The mildly amusing but wholly unnecessary comedy ''40 Days and 40 Nights'' will probably cause San Francisco to experience an uptick in desperate male tourists. That's because this mindless sex farce makes Tony Bennett's second home appear to be the site of a supermodel convention.

But that city's surfeit of skirts is not enough to ease the troubled brow of Matt Sullivan (played by Josh Hartnett, who graciously lends his own perpetually troubled forehead to the character). After being chumped and dumped by his girlfriend, Nicole (Vinessa Shaw), Matt begins dating furiously. But on each date, the anguished Matt hallucinates that a big black hole is opening in the ceiling. As a result, Matt lives through metaphors that are second-rate and sexist, which is in keeping with the movie.

Matt's newly hedonistic behavior infuriates his brother, John (Adam Trese), who is two years away from becoming a priest and blasts Matt with tough love. (Any movie with a title like this one -- and it's a great title -- has to get religion in somewhere.)

After spending time with his brother, Matt decides to mend his carnal ways and give up sex for Lent.

''Lent is about sacrifice, growth through denial,'' John tells Matt, still somewhat skeptical about his brother's vow of celibacy. As is Matt's roommate, the girl-chaser Ryan (Paulo Costanzo), who has less confidence than John in Matt's ability to control his rampant libido.

Sure enough, temptation arrives when the newly abstemious Matt meets the right girl, Erica (Shannyn Sossamon), who is initially charmed and eventually puzzled by his behavior.

To complicate matters, the guys at Matt's office find out about his vow and begin betting on his ability to keep it, as well as posting news of Matt's progress on the Internet, just to place ''40 Days'' firmly in the cartoon tradition that the late Chuck Jones left behind -- although Jones did a much better job of providing fully rounded characters.

Mr. Hartnett is suited to comedy, using that fixed expression of his for laughs; he seems to peer off into the distance at an unbelievable situation and his tiny eyes pop wide open when he realizes that it is his predicament that is coming into focus. His callow politeness is exaggerated into passive-aggressive behavior.

If Mr. Hartnett finds a way to keep working in comedy, he'll do well. And ''40 Days'' will probably encourage that because it will be a hit. Mr. Hartnett matches up with Ms. Sossamon, and not only because their eyebrows signal that they are both Vulcans. She is a more direct performer; what she does is not acting -- yet -- but she can look as if she's listening. Her approach contrasts hilariously with his gentility.

''40 Days'' is one of those movies in which the premise -- can Matt keep his vows despite carnal temptation? -- is reasserted in every scene, presumably on the assumption that the audience might doze off or receive urgent pager messages. The constant reminders -- the equivalent of grandly clearing one's throat before coughing -- have the unusual effect of throwing off the timing of the dialogue, some of which is quite funny.

The screenwriter Robert Perez tries to slip in some politics by having Matt's female co-workers protest that he is usurping their power by holding out. And there it is, folks, ''Lysistrata'' for the readers of Maxim.

''40 Days'' is set at one of those dot-com workplaces where no one does anything but talk about the sex lives of others. So, in its subversive way, ''40 Days'' could aspire to be a searing social satire about indolence in the workplace and . . . oh, never mind.

At heart, all the film wants to do is have fun, in the manner of Miramax post-teenage crowdpleasers like ''She's All That,'' starring Freddie Prinze Jr.

Mr. Hartnett is a better actor than Mr. Prinze, and most of the cast members know how to deliver a line. The stunningly talented Maggie Gyllenhaal does wonders with the stock role of the knowing best friend, keeping up with Joan Blondell and Elizabeth Perkins before her. And as the office lizard Chris -- the kind of sleek nerd who clearly spends every off hour prowling thrift stores for his seriously groovy collection of windbreakers -- Glenn Fitzgerald could hold his own with the young Jack Lemmon.

The director Michael Lehmann and his casting director, Joseph Middleton, have done a superb job of populating ''40 Days and 40 Nights.'' But the picture feels like a retread, right down to making an embarrassment of the 40-ish married boss, played by Griffin Dunne, whose sex life is over.

Someone is under the impression that each generation needs its own ''Love, American Style,'' which might not be wrong. It was bad enough when sitcoms used to pilfer the stories of romantic comedies. Then things got worse when movies were made from old television series.

But when we get to the point of movies shoplifting the narrative of television episodes for grist, what does that mean? ''40 Days'' burbles happily and thoughtlessly through a premise that supplied the plot for an episode of ''Seinfeld.''

''40 Days and 40 Nights'' is rated R (Under 17 requires accompanying parent or adult guardian) for strong language, sexuality, brief nudity and the kind of bad-boy behavior generally described in the letters section of Playboy magazine.

40 DAYS AND 40 NIGHTS

Directed by Michael Lehmann; written by Robert Perez; director of photography, Elliot Davis; edited by Nicholas C. Smith; music by Rolfe Kent; production designer, Sharon Seymour; produced by Tim Bevan, Eric Fellner and Michael London; released by Miramax Films. Running time: 92 minutes. This film is rated R.

WITH: Josh Hartnett (Matt), Shannyn Sossamon (Erica Sutton), Vinessa Shaw (Nicole), Paulo Costanzo (Ryan), Adam Trese (John), Maggie Gyllenhaal (Sam), Glenn Fitzgerald (Chris) and Griffin Dunne (Jerry).