DAVID MAMET is a 29-year-old Chicagoan who began his theater career as an actor, turned to teaching and within the last year has inspired an admiring cult as playwright.

"The Mamet Plays," introduced in Chicago and now on view at the West End Theater, are two short, highly amusing one-acters, "Duck Variations" and "Sexual Perversity in Chicago." They won off-Broadway's Obie award last year. His "American Buffalo" now on Broadway, is deemed by some New York critics as the season's best American play. Recently opened in Chicago to considerable admiration is "A Life in the Theater," which I have read but not seen.

One welcomes Mamet - but with decided reservations.

As the season wore on with not a single dramatic smash, some New Yorkers, fearing to wait any longer, decided in February that Mamet was the playwright the theater capital has been waiting for. But in March "The Shadow Box" opened just in time to cop the Pulitzer for Michael Christofer.

In "Duck Variations" we listen while two old Jewish gentlemen, with increasing vagaries and errors, try to top one another in revealing all they know about ducks, which is virtually nil. The setting is a park and their observations from the bench are phrased with such apparent exactness of expression that we smile with a kind of superiority but also with affectionate recognition. Their nonin-formation about ducks tells much about their lives.

Perversity, not perversion, is the central emotion of "Sexual Perversity in Chicago," with four office workers, two young men, two young women, awkwardly peforming the ritual of testing whether to make relationships permanent. Again, the talk is uncannily credible - words carelessly spilled or wrenched from hearts and minds that find living a solo that is vastly confusing, consuming and chancy.

The characters in both plays are not merely alive but, as important, likeable. The two old benchwarmers have the edgy mellow of age; the zesty adventurousness of youth quickens the quartet of office workers. Under the direction of Albert Takazauckas, the playing in both works is splendid.

Seeing these two plays and relishing the sparkle of the dialogue, one gets added pleasure from anticipation of Mamet's future.

I found the later "American Buffalo" emphatically disappointing because of its lack of plot and empathetic characters.

Here we have Donny, a junk shop owner who is baffled that a customer has just bought an "American Buffalo" five-cent piece for $50. He reasons that the man must be a coin collector, dispatches Bobby, a young dope addict, to tail the customer and discover where he keeps his valuables. With the help of Teach, who seems to drive a taxi, and a fourth character who never shows up, Donny plans a profitable heist.

None of the three characters has any quality of empathy. Mamet has been praised again for his dialogue. Donny's verbiage is filled wih allusions or words inaccurately chosen. Besides being a beehive of plots and plans that never come off, Donny in one of those lowlifes who can't say three syllables without inserting four-letter words. Bobby says little and Teach's gab is a variation on Donny's pretentions. The whole scheme will fall through and only Bobby is damaged.

What I find alarming about the play's reception is that some read into it the story of Watergate and "the boardrooms of American industry." Mamet quotes Veblen on "the myth of American business . . . nothing other than the sheerest mask for predatory behavior." What Mamet seems to be saying is that these ignorant, do-nothing lowlifes are no different from the wheels who get things done from corporations to the White House.

This is a far-fetched effort to intellectualize something out of nothing. Furthermore, the allegorical reasoning doesn't hold up when you consider that plots hatched in boardrooms and Oval Offices do not just peter out. They are carried through to tragedy. So what happens to all this "he means Watergate" business when nothing is the result? Praise for something not done is not helpful to maturing playwrights, so I worry about Mamet.

"A Life in the Theater" could well be affecting in performance. Mamet's two characters are actors, one older, but the younger as insecure in other ways. He imagines them as working acquaintances in various unworthy plays. There is the obsessiveness and insecurity of the profession and, as in the West End plays, a slice of life.,

But is this plotless presentation of characters enough? I think not. For a play to be satisfying something has to happen. Is Mamet simply stating that in most lives nothing really does happen? If so, why must we watch it not happen? Doesn't narrative drive have a vital part of a play? Too, this notion that people are utterly incapable of determination strikes me as cavalier.

Whatever, as you'll see so pleasurebly at the West End, Mamet can write shrewdly observed characters and imagine likable people in bizarre situations. Soon, I hope he'll find a story to tell us.