"Stay As You Are" is a two-year-old Italian triviality about the brief love affair between a middle-aged man, Marcelo Mastroianni, and a sultry coed, Nastassia Kinski, who may or may not be father and daughter.

It was brought in on short notice to fill a few booking gaps. One of the gaps was created by the failure of Taylor Hackford's appealing pop music melodrama "The Idolmaker" to attract business to the West End Circle. Could patrons have been waiting for this worthless substitute?

The incest motif in "Stay As You Are" is such a flimsy tease that director Alberto Lattuada can exploit it effectively only for dubious laughs. dA lanscape architect from Rome, Mastroianni encounters Kinski by chance in a formal garden in Florence and gives serious consideration to taking her up on hints of instant gratification. Temporizing, he seems to miss his chance; the impatient nymph spurns his company for a lift with a kid on a motor scooter.

Nevertheless, allis not lost: The girl, called Francesca, leaves her phone number on a piece of note paper. The smitten architect, Guido, who has an estranged wife and wayward daughter ("Want to hear a little problem? I'm pregnant.") to brood about, attempts to renew the acquaintance the following day. Before he can speak to the girl again, he runs into an old buddy inserted strictly for expository background. To your amazement as well as Guido's, he knows the girl and explains that she's the daughter of Guido's old college flame. In fact, it's widely suspected that this brazen cutie is Guido's illegitimate daughter.

Although Mastroianni, looking benign and comfortably paunchy, never seems to have serious erotic designs on Nastassia Kinski, he does his best to put on a worried fact at this disillusioning rumor. He brightens up when the girl suggests a visit to her father, a horse trainer. At the end of a convivial visit, in which Mastroianni gets a delightfully gratutious bit of business where he starts to tell a joke and then can't recollect the punch line, the trainer confides that he's really a foster father, Francesca's real dad being a secret that her mother preferred to take to her grave.

Inhibited by doubts again, Guido tries to resist Grancesca's entreaties, but she breaks down his resistance with the following irresistible argument: "Can't we just live for the moment? That's all that matters to me. You're thinking of things that will make us suffer. I've always wanted a fatherly type, and now I have him."

Despite a reverential consummation, with nude torsos decoratively crossed and intertwined on silky coverlets, and a playful morning-after, highlighted by the frolicsome lass' presentation of her saucy behind to be affectionately bitten (probably the movie's only claim to enduring fame), the affair seems to dwindle away in bittersweet regret. Don't ask why. There's no more justification for it to end than there is for it to begin. A perfectly conceived affair in its utterly insignificant way.

The movie is notable only as a launching pad for Kinski, a precocious sex bomb of 16 or 17 at the time the film was made. Once commonly identified as the daughter of German actor Klaus Kinski, who played the title roles in Werner Herzog's "Aguirre, the Wrath of God" and "Nosferatu," she is now commonly identified as the latest protegee of Roman Polanski. She plays the title role in his movie version of Hardy's "Tess of the d'Urbervilles," which may wend itw way stateside one of these years.

Although undeniably fetching, Kinski trades so heavil on her boldly challenging eyes, pouty mouth and naked nubility that she would degenerate into a seductive nuisance on very short acquaintance. If there's some erotic mystery left in this juvenile siren, Polanski will have his work cut out for him demonstrating what it is. Unless someone works wonders with her, "Stay As You Are" may be a springboard to obscurity.