GROWN-UPS - Fridays and Saturdays (except August 3 and 4) through Sepember 8 at New Playwrights' Theater, 1742 Church Street, N.W. Call 232-1122.

"Grown-Ups," the newest weekend fare at New Playrights' Theater, may well be the funniest cast of characters ever assembled on stage in Washington. As writer-director Tim Grundmann would have it, these zany adults speak the eloquent language of American satire in a highly polished collection of sketches.

The multi-talented Grundmann also supplies the musical accompaniment for the hour-long revue, and he speaks an exuberant line or two as well. Yet much of the credit for the success of "Grown-Ups" must be shared by the excellent and only players, Dana Vance and Jan Frederick Shiffman, whose improvisations were a springboard for the production. The pair intrepret all the roles with zest and energy.

The group's final product is concise and unequivocal, finding insights and amusements in a variety of circumstances. For example:

"Backstage" an actor and actress discover that each knows the other's role better than their own.

In the express lane at the "Grocery Store" an elderly patron feigns ignorance of the 10-items policy as she unloads her heavily laden cart.

The deal is off at the "Adoption Agency" when the father-to-be finds out that the premium he expected to get along with the baby - a tape recorder - is no longer given out.

"Rubbers" discovers a customer in a drugstore examining an extra-large prophylactic decorated with a detailed study of the Sistine Chapel.

This pastiche of perspectives leaves very little unsaid but allows ample space for broad interpretation. "Birds gotta chirp, folks gotta burp," observes one Grundmannian character in a moment of truth that probably gets quite close to the heart of the matter.

Like many offerings at New Playwrights' Theater, "Grown-Ups" is scheduled for a relatively brief run. See it soon. CAPTION: Picture, JAN FREDERICK SHIFFMAN AND DANA VANCE, STARS OF NEW PLAYWRIGHTS' "GROWN-UPS."