Not content with continuing their own string of gross-out comedies, the Farrelly brothers–Bobby and Peter–have set up a production company apparently designed to assist others to crank out similarly low-brow efforts. It’s probably not a wise idea, since the law of diminishing returns can soon set in. (As proof one need only consider the career of Mel Brooks, which started with a string of wonderfully over-the-top farces but quickly declined into has-been territory; could anyone stand to sit through “Spaceballs” or “Life Stinks” even once?)

The rule comes into play with the first offering from Conundrum Entertainment, as the Farrellys’ outfit is called. Written by Peter Gaulke and Gerry Swallow, two stand-up comedians of modest accomplishment, and directed by J.B. Rogers, who served as A.D. on the brothers’ own flicks, it’s titled “Say It Isn’t So,” but it might as well be called “There’s Something About Jo.” Mimicking the basic plot of “Mary,” the plot follows a fellow named Gilbert Noble (Chris Klein), who falls madly in love with a ditzy blonde, here named Josephine (Heather Graham), only to discover, to his horror, that she’s apparently his sister. Later, after she’s gotten engaged to another guy, he finds that they’re not really related, and so he goes off on a desperate odyssey to woo her back. His efforts are aided by a wild-eyed pilot named Dig (Orlando Jones), who happens to be a double amputee, bu are frustrated by his gold-digging false mother Valdine (Sally Field), who wants her daughter to wed the rich shark she’s engaged to. Much mayhem and slapstick ensue.

The pseudo-incestuous element aside, the similarity to the Farrellys’ previous smash is all too evident, and it’s not surprising that things aren’t as funny the second time around: it’s as though Gaulke and Swallow had determined to repeat all the major elements of the formula that made “There’s Something About Mary” so successful, with a twist so faint that only the densest viewer would fail to notice the similarity. Once again the hero is a obsessive but essentially mild- mannered sort who undergoes a succession of humiliations in his effort to get the girl he loves; as played by Klein, he’s more of a sweet-natured simpleton than Ben Stiller was, and he’s manhandled even more brutally, but those are but matters of degree. The object of his affection is a cheery but not too swift damsel, who (in her inept barber phase) cuts Gilbert’s hair in a way that can’t help call to mind Cameron Diaz’s unforgettable “do” as Mary. There’s also a rival, here Jack Mitchelson (Eddie Cibrian), a phonily understanding fellow who’s also a drug dealer, but he’s no match for Matt Dillon’s supremely sleazy private eye Pat Healy. Some gross animal humor is added, as the recipe demands: here, Jo’s randy cat substitutes for Mary’s frantic dog. One could continue the comparisons indefinitely: Stiller’s Ted Stroehmann endures a painful and embarrassing zipper stick, and Klein’s Gilbert gets his ear sliced off (in a shot that’s really quite revolting); one of Mary’s suitors was a phony paraplegic, and here much humor is based on the condition of Jo’s father Walter (Richard Jenkins), who sits incapacitated in a wheelchair as a result of a stroke. Eventually one actually grows tired of counting up the mildly-altered duplications. And “Mary” isn’t the only Farrelly model the present filmmakers have ransacked: if a guy got a chicken stuck up his derriere in “Me Myself and Irene,” why wouldn’t it be a hoot to watch poor Gilbert parade around with his arm embedded in a cow’s behind? By its close, “Say It Isn’t So” has come to seem little more than variations on the themes of its producers’ earlier movies; and if imitation is the sincerest form of flattery, it shouldn’t be forgotten that flattery is, by and large, a bad thing.

Nonetheless the picture isn’t a total loss. Klein recaptures some of the naive charm he exhibited in “Election” and “American Pie” but lost in “Here on Earth.” Graham is not only pleasant to look at but personable, too. Jenkins, almost unrecognizable until the closing reel, gets some rude laughs as an ill-tempered patient. And given the script’s scattershot approach, it’s inevitable that some of the crude humor should hit its target. Technically the picture’s fine, too, and the soundtrack makes good use of a nice collection of pop tunes.

On the other hand, many of the jokes as so tasteless that they’re actually repulsive, and they frequently involve violence that’s awfully unsavory. Field is standard-issue white trash; even her climactic comeuppance doesn’t raise one’s spirits. And Jones mugs so furiously as Gilbert’s buddy Dig that he seems to have stepped out of an old minstrel routine.

If the Farrellys intend to continue producing other people’s work, they should seek out projects that show a bit of imagination and don’t just try to parrot their own successes; even given an audience drooling for this sort of raunchy farce, the market can become oversaturated very fast. As for “Say It Isn’t So,” it isn’t so hot.