SALEM, Ore. — ''Sometimes it's a clutch of grade school teachers being led on a tour by that fool Public Relation man who's always clapping his wet hands together and saying how overjoyed he is that mental hospitals have eliminated all that old-fashioned cruelty. 'What a cheery atmosphere, don't you agree?' ''

--''One Flew Over the Cuckoo's Nest''

''You have to understand that there's a busy street outside,'' said our public relation man - a movie publicist - as he unlocked the door to the men's geriatric ward of the Oregon State Mental Hospital. In the corridor, old men shuffled by in paper slippers and disheveled dress, or they looked out, whey-faced and wary, from rows of cots in rooms without front walls. One patient handed out welcome presents - cigarette butts from a nearby ashtray - to the clutch of touring reporters. A few others stood uncomprehendingly before the sign at the entrance to the dayroom: ''Today is: Tuesday. The Weather is: cold. The next holiday is: Easter. The next meal is: dinner.''

''Are there ever romances between the men and women in geriatrics?'' the woman from Photoplay magazine wanted to know.

The reporters were here in Salem on a press junket (a word that irritated public relation, who kept calling it ''our Northwest excursion'') to watch the on-location filming of Ken Kesey's cult novel ''One Flew Over the Cuckoo's Nest,'' with a cast that included Jack Nicholson as R.P. McMurphy, the freewheeling con man who leads a revolt in the mental ward, and several dozen patients as extras. At first glance, the difference between the patients and the actors who play the patients - a group carefully selected for their eccentric manner - seemed to be that the former tried desperately to appear san and the latter didn't.

Our path into the ward was suddenly blocked by a spreading puddle of urine. ''Let's everyone detour over to ceramics,'' public relation announced. Ceramics turned out to be even duller than you might expect, so we headed back to the security ward in the main building, a three-story, yellow-brick structure built in the 1880's.

The security ward housed the criminally insane, and there the doors were barred and the entrance guarded by an electric eye. In one room of the women's section several aides were standing around a reclining patient, one of them pressing an oxygen mask against her face. ''Probably a cardiac arrest,'' public relation said. ''Let's move along quickly now.'' Later, a nurse admitted that the woman, a particularly recalcitrant patient, had just been given an unscheduled electroshock treatment.

Everything was business-as-usual in the men's maximum security ward, which had certainly not been the case when the ''Cuckoo's Nest'' company filmed a scene there for the first - and last - time a couple of weeks earlier. On that occasion a careless grip had left one of the protective screens open after running cable in from the outside, whereupon a young patient had jumped through the window backwards, falling three floors and severely fracturing his shoulder. The Salem papers had had their fun with the incident, one of them headlining its front-page story: ''One Flew Out of the Cuckoo's Nest.''

That was the only regrettable incident in two months of filming, according to Dr. Dean Brooks, the hospital's director. As part of their tour, the reporters spoke with Dr. Brooks, a handsome silver-haired man who looks like one of those central casting GP's who sells aspirin on television. Dr. Brooks in fact played the role of Dr. Spivey, the weak-willed hospital administrator, in the film. He told us that some of the hospital staff had objected to having the film made on location (the novel is set at Oregon State Hospital) for fear that it might infringe on the patient's right to privacy and give the place a bad reputation. Their concern stemmed from the fact that Kesey's novel is a savage satire of life in a mental asylum.

Dr. Brooks, however, felt that the therapeutic and financial advantages for those patients who got to work on the movie - as actors, technicians or maintenance people - far outweighed any disadvantages, although he insisted that the film be set in 1963 instead of the present and that a disclaimer be included saying that it is not a factual representation of life in a mental ward. ''I just hope people realize that this is an allegory about how a man can be caught up in the System and allowed to undergo electroshock and a lobotomy,'' Dr. Brooks said. ''Why, except for the one that was done two years ago, we haven't had a lobotomy in this hospital since 1958.''

That evening there was a press dinner at a local restaurant. I spent some time talking with Michael Douglas, the young star of television's ''The Streets of San Francisco'' and co-producer of the film, which United Artists has scheduled for release next fall. Kirk Douglas, Michael's father, had played McMurphy in the original Broadway stage adaptation of ''Cuckoo's Nest'' in 1963 and had bought the screen rights from the author then. ''Kesey's been bad-mouthing us recently because we rejected his screenplay and he thinks he got a raw deal financially,'' Douglas said. Since Kesey lives in nearby Eugene and is something of a local hero, his hostility toward the producers had not done the film company much good in establishing friendly relations with the townspeople, a situation that was exacerbated when he told a Salem reporter that in Newport, Ore., where another Kesey novel (''Sometimes a Great Notion'') was filmed, ''the Hollywood dream-makers made a lot of promises, got three women pregnant and, suddenly, they were gone.''

The town's collective eyebrow was raised again when the producers asked Mel Lambert, a Salem used car salesman and sometime radio announcer, to round up a number of particularly grotesque-looking locals to play extras in a scene in which McMurphy is locked up in the maximum security ward. Lambert placed an ad in the local papers which asked: ''Do You Have a Face That Scares Timberwolves?'' Despite subsequent editorials protesting the insult to the local citizenry, Salemites trooped down to Lambert's car lot by the score. Several would-be stage mothers came with their offspring in tow, including one who pushed her perfectly presentable 12-year-old son forward and announced: ''Here's my monster. If you can't use him, you can't use anyone.''

We were still standing around the restaurant, picking from the canape tray, when Jack Nicholson walked in wearing a check flannel shirt, jeans and a two-day stubble. At first sight Nicholson, who is on the short side, has thinning hair and speaks with that celebrated Jersey twang, doesn't seem to be what Kesey had in mind for the red-headed, broad-shouldered, hell-raising McMurphy. But he's got those deadly cobra eyes, a smile he can wield like an ice pick and a manner that suggest violence under temporary restraint. He's also got McMurphy's quick wit. As he entered the room, a waitress dropped a highball glass, which burst on the floor like a pistol shot. ''And stay out!'' Nicholson yelled to no one in particular.

At dinner, Nicholson kept raising his glass in silent toasts to the butterfingered waitress whenever she passed by. ''Best thing I've seen so far in Oregon,'' he said. When she came around to tie his lobster bib on, Nicholson looked up at her backwards and flashed his famous smile. ''What're you doing later?'' he asked half-heartedly, throwing his arms around her and bringing her closer to him.

''Hey, Jack, that doesn't come with dinner,'' Douglas said.

''It does for me,'' said Nicholson.

The next day I visited the set, actually an abandoned wing of the hospital. Most of the filming had taken place in the ward where McMurphy leads his ill-fated rebellion against Big Nurse and the Combine. But today there was a break in the drizzly Oregon weather and a sequence was being shot outside on the maximum security ward's basketball court, an area surrounded by a double row of cyclone fencing topped with barbed wire. In preparation for the big game between the patients and the hospital staff, McMurphy was trying to teach Chief Bromden, the huge Indian who adapts to life in the ward by pretending to be deaf and dumb - a literal wooden Indian - how to dunk the ball. ''We're gonna play a great American game,'' McMurphy/Nicholson was saying, coaxingly. ''It's called put-the-ball-through-the-hoop.''

Bromden was being played with all the emotional range of a brick wall by Will Sampson, a 6-foot 4-inch, 235-lb. full-blooded Creek whom car salesman Lambert - instructed to come up with the biggest Indian he could find - spotted at an Indian trade fair in Yakoma, Wash. ''Now that we've found Will they ought to redesign the nickel,'' public relation whispered to me.

Milos Forman, the director of ''Cuckoo's Nest,'' made Nicholson and Sampson do the short scene over and over, twice because ''Voit,'' the band name on the basketball, had been too prominently displayed. (''Keep it,'' public relation muttered, ''I'll bet we can get a tie-in.'') After the ninth retake, Nicholson left for his trailer in a mild huff and Forman turned his attention to shooting reaction shots of the basketball game on the sidelines by some of the other character actors who were playing the ''chronics'' and ''vegetables'' in the film.

Forman, who wore a red scarf and sucked incessantly on a pipe, had made ''Loves of a Blonde'' and ''Fireman's Ball'' before emigrating to the United States from Czechoslovakia in 1969. He has a reputation for encouraging improvisation among his actors (often nonprofessionals- and is noted for his light, comedic touch. He had chosen Haskell Wexler as his director of photography, but fired him when the veteran cameraman insisted on a starkly dramatic approach.

At the moment, Forman was filming a reaction shot of Colonel Matterson, who rode up and down the sidelines of the basketball court in a flag-draped wheelchair yelling: ''Retreat! Retreat! Send the cannonball into the citadel!'' The Colonel, an addled ex-Rough Rider lost in a military dotage, was played by Peter Brocco, a 73-year-old actor who works as a potter between bit parts in films and plays. Brocco modeled his character partially on a paranoid-schizophrenic he met at the hospital named Leo Fischer, who styled himself LÀeon de Fischer, Emperor of the World, and babbled nonstop about how he had divided the world into 12 nations and had placed one of his relatives on the throne of each. Fischer is a gentle man but he had, understandably, great family pride, and when Brocco had asked him what he thought of the British royal family, he had hissed a single word: ''Interlopers!''

After spending the afternoon watching reactions to a phantom basketball game, I decided to attend a real game that the patients' team was playing at a high school outside of Salem. Some of the actors, I was told, had taken an interest in the team, although its season record of 12 consecutive losses seemed to leave little enough to cheer about. Unfortunately, I had trouble finding the school and when I finally arrived 45 minutes late, the team had already showered and dressed. ''You missed a good game,'' the coach called out. ''We only lost by 40 points tonight.''

Movie sets and mental hospitals have this in common: endless waiting around, time filled by playing cards, shooting pool and watching a lot of television. And there is an inevitable sense of disorientation. For example, on the next day filming started at 6 p.m. and the cast had ''lunch'' at midnight. During the meal, I talked with a patient named Gordon, a tall, good-looking fellow (he played center on the hospital basketball team) who had been committed to the hospital twice for rape. One of the few patients who had a full-time job with the film company, Gordon had helped build the set and now worked with the custodial crew. He had found his initial suspicion of ''Hollywood types'' unjustified. ''Usually I think I have to be some one else for people to like me, but these people have accepted me that way I am.''

That day was co-producer Saul Zaentz's birthday, and after ''lunch'' Jack Nicholson, hopping from table to table still dressed in McMurphy's dirty scrub suit, began pouring champagne for everyone while Scatman Crothers, who play a hospital orderly in the film, plucked a down-home rendition of ''Happy Birthday'' on his ukulele. There was much back slapping and general hilarity in the room. ''These people aren't so weird after all,'' Gordon said.