The trouble with Carrie Sarah Jessica Parker has spoiled the delicate chemistry of "Sex and the City" by turning her once-flawed character into a boring uptown bombshell -- and by refusing to get naked.

"Sex and the City" begins its sixth and final season on Sunday, and if you were to take the ads for the series at face value -- the ones that have shown up in bus shelters, phone booths and magazines in New York and elsewhere -- you'd think the show had only one star. Those ads feature Sarah Jessica Parker as Carrie Bradshaw, the Manhattanite sex columnist whose adventures in the world of urban dating (not to mention shopping) have always been the centerpiece of the show.

The ads, done in moody, soft-focus black-and-white, give off the perfume of tasteful, risk-free glamour. Parker-as-Carrie looks like a '50s Italian movie star in glossily tousled blond curls (with roots just sooty enough for street cred); her dreamy eyes are rimmed with Cleopatra-via-François Nars kohl, her lips molded into a sexpot pout. This Carrie is more together than ever, at least in the magazine-ad kind of way -- you can just tell she's sexier, softer, kinder than before. I'll bet she still smokes, but she probably doesn't wield a butt with the absent-minded, aggressive fierceness of the old days; she now has a heightened awareness of how elegant the cigarette looks perched in her hand. She's more confident and more dazzling than ever, a woman striding toward her 40s with a wan smile and a tight butt.

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Would the 1998 Carrie Bradshaw be able to stand the 2003 version?

Ensemble comedies are tricky things, and the longer an ensemble comedy survives, the rockier the terrain gets. For most of its five previous seasons, "Sex and the City" has been the brightest, most stylish, most consistently entertaining ensemble comedy on television. That's largely thanks to Parker, but not solely: Her three costars have always been as integral to the show's pleasures as she is. Kim Cattrall is Samantha, a woman who dates around, happily, without commitment, enjoying a sex life something like the one Hugh Hefner envisioned for himself and his fellow playboys back in the '50s. Cynthia Nixon is Miranda, the sensible corporate lawyer who's tougher and more blunt than most of the men she dates, but who has a core of kindness that the others can't match. And Kristin Davis is Charlotte, the sweet but hardly dumb brunette who believes in true love above all, although she doesn't underestimate the value of real estate.

The four actresses have proved to be a formidable and beautifully integrated team. For the first four seasons, at least, they complemented one another as if each of their careers had been headed toward this single focal point all along. They must be a joy to write for, considering their rapport is almost palpable: They pick up on one another's cues with ease, bolstering their respective strengths and rendering their weaknesses invisible.

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But sometime during the fourth season, things began to change, most significantly around Carrie's character. Parker's Carrie has always been the ringleader, and at first, especially, it was easy to see why.

For one thing, the show was based on the work of real-life New York writer Candace Bushnell, with Carrie her fictional counterpart. The role represented Parker's biggest break, after years of being a well-regarded actress but not exactly a star, and it was a break she deserved: Parker is one of the most gifted comic actresses of her generation. Her timing is sure and sharp. She has a knack for physical comedy. And her beauty is more classical than classic: The contours of her features are noble and good-natured. She has the kind of face you'd see on a Roman coin.

All four characters have changed since the show's inception -- they'd have to, or what would be the point? But Parker's Carrie has changed the most, perhaps not coincidentally since Parker became one of the show's executive producers a few seasons back. Since the show's debut, Miranda has had a baby; Samantha has attempted a committed relationship; Charlotte has gotten married and then divorced. And plenty has happened to Carrie, including an on-again, off-again romance with the charismatic and mystifying Mr. Big (Chris Noth) and a messy almost-marriage to the dopey dreamboat Aidan (John Corbett).

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But somehow, the story lines written for Carrie now seem bigger and bolder, even as her mistakes and missteps don't carry the same weight as those of the others. There's something vaguely superior, or maybe even not so vaguely, about the way her character eases, time after time, into the show's most glowing light. Parker has changed as an actress, and not wholly for the better: Her line readings can still be wonderful, but there's something stiff and self-conscious about her, particularly as her character has shifted from a loopy, smart, sharp-witted urban beauty to a chic, classy, uptown-style babe. She seems to have slipped all too comfortably into the role of boring bombshell. She was infinitely more likable, and more interesting, as an awkwardly confident city girl.

Miranda, Samantha and Charlotte have grown more complex and more intriguing as characters; Carrie has simply grown more polished. And there's one more thing that sets Carrie apart from her cohorts, and Parker apart from her costars: Of the four, Parker is the only one who routinely wears a bra or "bedsheet bandeau" during love scenes.

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The unspoken message is, It's OK for them to take their clothes off, but you won't catch me doing it.

I've often heard real, live single New York women complaining about "Sex and the City": "Five hundred bucks for a pair of shoes -- no way!" "That walk-in closet is the size of my bedroom and living room combined!" "My friends and I never talk about vibrators at brunch, nor would anything as sickening as a cosmopolitan ever cross our lips!" And don't forget the ever-popular "I wouldn't be caught dead wearing a tam-o'-shanter at an outdoor cafe!"

OK, I admit that Carrie's Season 5 tartan tam did come close to snapping the bra strap of suspended disbelief. (Carrie's outfits, which have become more bizarrely affected with each passing season, are too much of a galloping horror to go into here.) But since when is farce supposed to be realistic? Even if the show were true to life, its value would never lie in how accurately it portrays the reality of single city women. The show's grand joke is that while just about every city woman wouldn't mind some degree of glamour and sophistication in everyday life, the city's job is to prevent glamour and sophistication whenever possible -- there must be a budget allocation for it.

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Strategically placed wind tunnels in various locations around the city are waiting to blow your dress up around your shoulders whenever possible, preferably as you're passing a construction site. When you go out for your Times in the morning, you may very well brush by a man peeing against a wall. (For some strange reason, it never happens if you're buying the Post or the Daily News.) That careering city bus probably isn't going to hit you, but it is likely to splash nasty puddle water on your Gaultier tutu the very first day you wear it. The nerve of the place!

The city -- and society at large, which of course encompasses the dating world -- demands civilized behavior from us, and look what we get in return. That's a delicious setting for the bumpily unfolding saga of these four women -- all of them in their mid-30s or thereabouts and all of them good-looking, independent-minded and reasonably successful at their careers -- as they try to manage the unruliness of mere living.

"Sex and the City" is neither a window into real life nor, heaven forbid, a model of how to behave or dress when you're a young woman living in the city. Its sophistication lies not in the fact that these women lead sophisticated lives; in fact, the noisy jangle between the lives they're trying to build for themselves and the lives they're actually getting is what makes "Sex and the City" so delectable.

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The writing doesn't represent the way people talk as much as the way we want to imagine people talking. "Sex and the City" is high comedy sparked with low, a modern comedy of manners in which it's perfectly appropriate to use common slang as well as forbidden words like "pussy" and "dildo." "Sex and the City" picks up threads of the dexterously clever tradition of Oscar Wilde and Noel Coward and adds dollops of low comic raunch. When the show is in high gear, both the writing and the acting show a deftness of touch and a love of the ridiculous, as well as a recognition that if our lives were exactly as we wanted them, they wouldn't be nearly so funny. Society does exist for a reason: It's there for us to make fun of, to chafe against, even if we freely admit to enjoying its trappings. In "Sex and the City," a silk gardenia on a jean jacket subs for a boutonniere on a dinner-jacket lapel.

Like all the "Sex and the City" actresses, Parker has been a joy to watch, straight through the first three seasons and most of the fourth. But in the fifth season -- or perhaps beginning near the end of the fourth, around the time Carrie broke up with Aidan for the second time -- Parker suddenly seemed to be trying too hard. Her look on the show became more polished, for one thing: Her bobbed hair had lost some of its appealing, wayward craziness; her eye makeup looked as if it were the result of a solid half-hour in front of the mirror, instead of the more natural "two flicks of the mascara wand" effect most of us busy girls make do with.

Parker's new look was definitely prettier: For better or worse, she suddenly seemed more like a conventional beauty. But as the season wore on, she seemed to have lost some of her spark, her eccentricity. Her mannerisms, and even the set of her mouth, seemed to change subtly, becoming more studied and less natural. You started to see the broad, goofy grin less and the practiced smile more. And tiny, weird aberrations suddenly came to the fore: Even when she does something as simple and as seemingly natural as pushing her hair back from her face, her movements seem calculated and affected. (I've noticed her using one erect forefinger to delicately and precisely push back the front strands of her hair -- the kind of thing I've only ever seen drag queens do, and even then, only onstage.)

The difference between the old Parker and the new is even more pronounced when you look at the first few episodes of the show, from the late '90s. The 2003 Carrie walks with the gait of a supermodel. She radiates glamour with a capital "G," a far cry from the woman who, just seconds after meeting Chris Noth's Mr. Big on the street for the first time, totters off on her stiletto heels, pulling down the back hem of her black ultramini with a few mighty tugs. For a split second, her legs threaten to fold under her; with a coltish toss of her head, she rights herself, averting disaster. Parker's trim figure is lovely, but her faintly gangly physicality (and, more specifically, her ability to put it to such good use in comedy) is what makes it truly beautiful. In that raven minidress, she's a chic black daisy on impossible stems.

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The new Parker might stumble on her stilettos now and then just to remind us she's still human, still a little bit like the rest of us. But it isn't nearly the same. In one of the first-season episodes, she and Miranda, Samantha and Charlotte sit around a table eating takeout, grumbling about how so many men seem to want only supermodels. Samantha's self-image is solid, but Miranda and Charlotte are tough on themselves. "Look at you two, you're beautiful!" Carrie exclaims, annoyed and impatient with their nonsense -- she refuses to coddle them. With one hand, she holds a copy of Glamour magazine aloft, making a remark about its near-perfect cover model. With the other, she picks a piece of food from between her teeth. And in between all this, she barely stops talking.

Now that's our Carrie. More recently, though, a certain prissiness -- something that was never apparent before -- has crept into her character. Maybe the writers (the show uses a revolving team of writers and directors, with a few regulars) are mostly to blame for that, but you can't help wondering if Parker hasn't had a hand in shaping her character's direction.

The most blatant example is an early episode of Season 5, in which Carrie accidentally walks into Samantha's office just as Samantha is giving an impromptu blow job to the overnight-delivery guy. Carrie flees the office, embarrassed. Later, Carrie makes a sneering wisecrack about the incident, and when Samantha accuses Carrie of judging her for being "loose," Carrie admits that she would never behave as Samantha does. Samantha is understandably hurt. And while Carrie does eventually acknowledge that she had been judgmental toward Samantha, there's something a little superior about the way Carrie sits so comfortably in the role of the good girl who would never act like "that." Even the way she learns her lesson is a little smug; Carrie is magnanimous enough to have seen the error of her ways. But you still wonder if, deep down, she doesn't think Samantha is just a little cheap.

Carrie's persnicketiness about what she will and won't do in her sexual life is underscored by the fact that Parker is the only one of the show's four actresses who won't do nudity. At one point or another, Cattrall, Davis and Nixon have all stripped down to varying degrees; Cattrall has been the boldest of all, which makes sense, since Samantha's sexuality is so integral to her character.

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But how many times have we seen Carrie in bed with this or that beau, making love well into the night with her bra firmly fastened around her torso, or with the sheet wrapped primly around her shape? Whether or not an actress does nudity is her choice to make. But when you're part of an ensemble of actresses as fearless as Cattrall, Nixon and Davis -- actresses who don't strip down wantonly, but who will do so when the script demands it -- there's something cowardly about Parker's effusive modesty. All actresses have to protect their image to some degree; but sometimes, in order to make a character seem real, you've got to bare more than your soul.

On the one hand, you could applaud Parker for her staunch principles. And again, her contract is hers to negotiate, after all. But there's a whiff of hypocrisy in the fact that she's the only member of an ensemble cast who will notably not take off her clothes. The show is called "Sex and the City," which means that it's sometimes going to be about dating, which means that it's at least occasionally going to be about sex (although, admittedly, the show is almost never as overtly about sex as its name would lead you to believe). In real life, sex often leads to nakedness. I wouldn't have wanted any other actress to play Carrie Bradshaw. But I think an actress's willingness to do nudity should have some bearing on the roles she chooses to begin with.

If a character is never seen naked, even when the setting would make nakedness seem completely natural and believable, doesn't that suggest that the actress playing that character is putting her image ahead of the demands of the role? Worse yet, I think Parker's unwillingness to strip down -- even in a modest way, as, say, Davis has done -- suggests an unspoken judgment about her colleagues.

An acquaintance of mine who worked as an editor of children's textbooks once explained the intricate rules of illustration to me: You can't show a bear wearing only a shirt, since that implies that the bear ought to be wearing pants, but isn't. By the same token, Parker's resolute modesty stands out, suggesting that her colleagues should be wearing brassieres or bedsheet bandeaus and simply are not. Parker obviously doesn't feel comfortable doing nudity. But the strictures she adheres to make us even more aware of the chances her colleagues willingly take.

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I freely admit to being tougher on Parker than I am on any of her colleagues. That's partly because, in the past year at least, it seems that Davis, Cattrall and Nixon have been overshadowed by the Parker spotlight, even though their work has been much stronger than hers. I feel particularly protective of Davis: Charlotte, with those kitty-cat cardigans and that fetching schoolteacher smile, was pure genius last season. I can't wait to see what she'll do in this last one.

But I still hold out hope that Parker will turn herself around. The show's writers have hinted that Season 6 will feature plenty of Mr. Big -- a good sign, since Parker has always been funnier and more relaxed with Noth than with any of her other on-screen boyfriends. Parker won my heart long ago, perhaps around the time she played the bendy, spacey SaNDeE* ("and a star over the E!") in Steve Martin's "L.A. Story." I want her show to be as resoundingly vibrant in the end as it was at its start.

I can't begrudge Parker the desire to enjoy being, at last, a conventional beauty. She can keep the perfect hair and the meticulous eye shadow and the weirdo-chic wardrobe, the closet full of Manolo Blahniks and the New York apartment that could comfortably house a family of four. Realism be damned -- humanity is all that counts. I just want to see her pick her teeth.