Joan Rivers does a walk-on (a run-on to be precise) in "Rabbit Test," a tiresome farce which gets her off-and-slumbling as a film comedy writer-director. She appears as a nurse supposedly rushing a vital organ to the operating room. "Hot colon coming through!" she shouts, affecting the style of a brash waitress at a crowded hash house. When she bumps into someone and the play colon fails to the floor, she picks it up, slaps it back on her tray and dashes off, muttering curses.

Rivers' behavior in this bit typifies her brusque, low-minded approach to attracting and amusing a movie public throughout "Rabbit Test." The script consists of a gruesome procession of predominantly sick and scatological "hot ones," which fall flat with dreadful consistency.

Unlike transvestism, male pregnancy doesn't seem to be a fruitful pretext for sustained humor on stage or screen. Try to list the comedies based on this gimmick that rank with "Twelfth Night" or Charlie's Aunt" or "Some Like It Hot" or even "Turnabout" and you end up with a black list.

Jokes about the indignities of maternity have been a Rivers specialty, but they're no longer fresh and the pretense of reviving them by making a virginal guy the butt of the jokes proves a lamentable fraud. Divorced from Rivers' personal experience and viewpoint, the subject of fertility turns out to have no comic potency.

The pregnant make function as her come-on wheeze in a grap bag of wheezes. It's a tribute to Billy Crystall that he emerges from his starring assignment in "Rabbit Test" still looking more or less fresh, attractive and dignified, because the role itself rubs your nose in indignities and vulgarities.

As the unfortunate hero's maniacal mom, Doris Roberts frequently yells for her long-lost husband, called Mel. During these outbursts it's difficult to resist the thought that Rivers must be involving the spirit of Mel Brooks. "Rabbit Test" owes much too much to the spirit of "Blazing Saddles." In the annals of overcompensatory anal-retentive joking, Rivers may have succeeded in carving out an even lower niche for herself.

For example, during one stupefying stretch, a reference to "Gladys Knight and the Poops" is followed by a sequence in which someone (maybe it was George Gobel as the president) tells the hero, "Compared to you, the moon walk was doo-doo," and then a sequence at the Vatican in which a prissy, irritable pope says that he never appears in public anymore because "the pigeons keep messing on my hat."

Enough ought to be enough, but not for Joan Rivers on this outing. Too unsure of her audience to be subtle and too unsure of her technique to be deft, she keeps flailing away.

Could a steady diet of Las Vegas and TV game show crowds have given Rivers a peculiarly distorted view of popular taste? If "Rabbit Test" doesn't betray a lack of respect for her public, it must betray a lack of respect for herself. A more elevated level of humor wouldn't hurt "Rabbit Test," which now looks inept as well as disreputable. Rivers doesn't justify this stuff as either inspired or liberating low comedy. At best it's infantile, and frequently infantile with a vicious streak, presumably the humorist's built-in defense mechanism agaist rejection.

Joan Rivers might be capable of perfectly agreeable and presentable movie comedy. She does have a distinctive sense of humor and abundant energy. It's fun to imagine her having a go at novels like "Once Is Not Enough" or "The Other Side of Midnight." What won't do is material as yuck and insulting as this.