Matt Stone went to Norway recently. It didn't really work out. "Holy shit, man," he says. "There was some asshole there telling me how America should be more like Norway. Literally, 'Well, in Norway ...' I'm, like, 'I don't care! That doesn't apply to America! It just doesn't! Dude, you have four million people. You're all blonde. You're all brothers and sisters...'"

He sighs. "Americans can't tell them our impression of them."

Too offensive? "To be honest, we don't have one. Norway is never in the news. I don't know what they do up there."

Despite what you may think, the co-creator of South Park, the show in which a bunch of rudimentarily drawn schoolkids have spent the last 12 years comprehensively dropping their trousers to their homeland, is proud to be called an American. "I like America," he says. "When I go overseas I spend a lot of time defending my country. America shouldn't work. There are 300 million people. Every day it hasn't evolved into people eating each other and race war. It's like 'OK! Let's count that as a success'."

Stone, 37, is sitting in his office in South Park Studios, a blue and purple-painted warehouse resembling a BT call centre in California's Culver City, five minutes drive from his house in Venice Beach. Together with his college friend Trey Parker, 39, he creates and produces every episode here, beaming them via satellite to New York by 11am on Wednesday mornings, for broadcast that evening. (These same episodes will be shown in the UK 48 hours later).

On Parker's office wall is a signed photo of Saddam Hussein, gifted to him by the US Army's 4th Infantry Division. During his time in captivity, Hussein was apparently shown the 1999 movie South Park: Bigger, Longer And Uncut, in which he's depicted as gay, and enjoying intercourse with the devil, repeatedly. "I have it on pretty good information from the marines on detail in Iraq that they showed him the movie," says Stone. "That's really adding insult to injury." In another room a whiteboard details all the plot outlines for the 14 episodes of South Park season 13, the first of which airs in 12 days' time. "An alien comes down," reads one note. "People farting a lot," reads another. And that's it. "Farting and aliens. Doesn't sound great, does it?" Stone guffaws, in his basey way. "But it'll be better than that. Next week we'll get a fire under our asses and start making it real."

Unlike The Simpsons, where each episode takes nine months to complete, South Park is made from scratch in a week - the 70-strong team sometimes pulling 22-hour days. That way Parker, who writes all the dialogue and directs, and Stone, who produces and serves as a buffer with Comedy Central, can respond to events with a speed second only to the news. They depicted Hussein's capture three days after it happened, parodied the Terri Schiavo right-to-die case the week Schiavo passed away and, last November, had Barack Obama in the White House 23 hours after the election. "If we worked on an episode for three weeks, we'd get really bored," Stone says. "You'd start hating it."

Stone and Parker say they're "equal opportunity offenders", and their broad anti-authority message (everyone's an idiot so look after yourself, basically) flourished under the Bush administration and America's swing toward conservatism and political correctness. While The Simpsons has settled into its own cosy groove, South Park has got crankier and crankier. "The people who make The Simpsons are not the same people who made The Simpsons six years ago, and they're not the same people who made it six years before that," Stone says. "You have a different ageing process. We're getting crusty, old and mean. They're always young and happy!"

Simpsons aside, precious little in pop culture maintains standards - let alone relevance or public interest - for 12 years. Today South Park's as good as it's ever been. "Matt and Trey have gone more and more towards the headlines; whether that's Britney or Obama," says Jill Offman, MD of Comedy Central UK. "That's served the programme extremely well. Even before we've had chance to absorb the news, they've turned it around and come up with profound social satire. They're always current. In that respect, they're untouchable."

Last season Cartman contracted Aids only to discover no one cared about the disease anymore. Britney Spears shot herself in the head; then was forced to record a comeback song. And when they woke to find the internet has dried up, the town evacuated to a Red Cross Internet Refugee Camp, desperate for porn. Naturally Stone denies they've ever had anything as grand as an agenda. "Maybe when we started there was a reaction against political correctness," he says. "But we weren't thinking about that. We were just 25 years old and trying to be funny."

Neither has ever voted. "Each election is a choice between a douche or a turd, so who cares?" Parker observed during 2004's US elections. They didn't feel differently this time? "My wife made me get a mail-in ballot and I brought it to work," Stone says. "But then I gave it to this girl and let her be my vote." Is that legal? "No; it's totally lame. We should have a whole recount 'cos of me!"

South Park has made Stone, the son of an economics professor and Parker, the son of a government geologist, two Monty Python fans who met at the University Of Colorado at Boulder where the former was making hand-held films like The Giant Beaver Of Southern Sri Lanka, featuring Godzilla-style rampaging beavers, and the latter was a maths nerd, extraordinarily rich. Together, their take-home share of the ad revenues for content aired on mobile phones and the internet alone is pounds 37.4m. The merchandising deals that gave the world the Mr Hankey Talking Bobblehead and Chef's Love Sausages top pounds 300m. Then there are the movies; 1999's Bigger, Longer And Uncut and 2004's Jerry Bruckheimer-riffing puppet extravaganza Team America: World Police, which grossed pounds 58m and pounds 35m respectively. "Team America was the worst time of my entire life," says Stone. "You go 'Oh yeah, let's go do a movie, that'd be awesome'. And it's so much work. Then it's 'Man, let's get back and do our show. This is a bunch of bullshit. It's a fucking headache'."

Given that Parker already owns eight homes, perhaps its no surprise they've arrived at their latest wheeze - giving South Park away for free. One hundred million episodes have been streamed since making their collected works available on their website last year.

A charitable view, though not one shared by Sky; who've just put the kibosh on UK access.

But South Park would hardly be South Park if they didn't test their employer's patience. There was the recent episode where they broke the swearing record ("shit" 162 times, as an on-screen counter helpfully confirmed). The episode where Tom Cruise was asked to "come out of the closet" 28 times ("It was the cover-up that screwed him," Stone says, of the resulting hoopla that allegedly found Cruise threaten to pull Mission: Impossible III promotion for Comedy Central owners Paramount, should a repeat air. And then deny it. "Made him look weird.") The episode - shown at Christmas - where a Virgin Mary statue miraculously bled from its bottom and was inspected by Pope Benedict XVI, who declared the effigy merely menstruating.

Only once have Comedy Central put their foot down recently, when Stone and Parker wanted to reproduce an image of Muhammad; the same one that prompted worldwide rioting after appearing in Danish newspaper cartoons. ("Danish cartoons? That's our competition," Stone grumbled. "The Danish?")

"I look back at the show sometimes and go: 'Really? We had the Virgin Mary bleeding out of her ass?' You don't think that's offensive? It's pretty fucking offensive. I think people have different standards for South Park than for everyone else."

As for striking a blow for free speech... "We take on all these big subjects and then people are, like, 'You guys did that thing in the closet with Tom Cruise!'" he sighs. "It's all anybody talks about."

Stone and Parker are contracted to South Park for three more years. "We're pushing 40, so one of these days we've got to quit," says Stone. "That will be sad. This is all I've done; all my adult life." South Park staffers describe their offices "like a fraternity that lives on", but every fraternity has to graduate sometime. Though Comedy Central still picks up the tab for twice-yearly writers' retreats to Vegas or Hawaii; days filled with drinking, strippers and other diversions, lately there's been fewer office pranks like the one where Stone forced an intern to eat six McRibs and four Starbucks lattes, then drink the resulting vomit. "It's not quite as crazy Animal House here as it used to be," he notes.

He and Parker have no more plans to dress up as Jennifer Lopez and Gwyneth Paltrow, drop acid and hit the Oscars, like they did in 2000. "We did that; now we're kind of old. Seasons three and four, I could stay up all night on coffee and cigarettes. Now I've got to have fruit and go on runs."

Anyone fearing a slide into squishy middle age probably needn't jump ship just yet. Together they're working on a musical called Mormons with the composer Robert Lopez, the producer behind warped puppet show Avenue Q. It's destined for Broadway, or Hollywood. "The songs are amazing," says Stone. "It's one of those great projects that are really fun at the moment. When we actually produce it, we'll hate it."

"And then we'll probably come back to South Park."

• South Park, Fri, 10pm, Comedy Central