LUNCH HOUR by Jean Kerr; directed by Mike Nichols; scenery by Oliver Smith; costumes by Ann Roth; lighting by Jennifer Tipton; produced by Robert Whitehead and Roger L. Stevens.

With Gilda Radner, Sam Waterston, Susan Kallerman, Max Wright and Jon DeVries.

At the Eisenhower theater through Nov. 1.

Shades of the past!

The list of "Lunch Hour's" ingredients could have been drawn up 20 years ago: one affluent, youngish couple with no apparent worries in life except each other: one extramarital affiar (genuine); a second extramarital affair (pretend, for revenge); and a well-appointed living room-cumveranda in which to cook the plot.

But first, let the mixture sit, and gradually add, for the aroma of the 1980s, several plants, one "dispose-all" reference, one "pritikin Diet" reference, and one Gilda Radner.

We pause. As any chef will affirm, it is astonishing what the presence or absence of a single ingredient can mean to even the most mind-boggling of recipes. Try to imagine "Lunch Hour" without Gilda Radner, for example, and what you get is . . . what you get is . . . But why must we wander down that nightmarish corridor? There is no need. The vital ingredient is here, present and reporting for duty: one instant Broadway star.

It is no simple task to understand how an actress can coax a huge laugh from a line like, "I've never fainted in my life -- not even when I smashed my hand in the car door." They should give foundation grants and extended sabbaticals to study theatrical phenomena like that.

In the meantime these spotty conclusions: Radner has notions about comic timing from which it can be inferred that she probably shared a room with a bazooka as a child. The Radner repertoire is in no way limited to the small screen, to the brief walkon, or to Baba Wawa, Lisa Loopner, Roseanne Roseannadanna and the rest of the "Saturday Night" menagerie. She is ready for the theater, if the theater is ready for her.

Jean Kerr seems to be ready. Although it is said that Kerr had never even seen "Saturday Night" or Radner when she wrote this play, the part of Carrie includes blank looks, off-the-wall digressions and an ultra-inferiority complex that no one else could possibly have brought to such absurd life. In the middle of her climactic confrontation with The Other Woman, for example, Radner rushes to the rescue of her rival's groceries, warning: "That bag's starting to make a puddle." And when she learns that The Other Man (i.e. The Other Woman's husband) is a marriage counselor who wrote a best-selling marital self-help book called "Settled Out of Court," Radner recalls, to her amazement, that she has read it. She got it from the Literary Guild, she tells the author. "You know, if you don't send the little card back, you get the craziest books!"

Now is as good a time as any to acknowledge that "Lunch Hour" has a plot -- and what a plot! Sam Waterston, the marriage counselor, is subjected to constant accusations of infidelity by Susan Kellerman, his wife. But in fact, she is the one having the affair -- with Jon De Vries, a playboy art collector. De Vries spills the beans to Radner, his wife, who spills them in turn to Waterston. Then they decide to dream up an affair for themselves to put their spouses back in line, only to discover that the dream is starting to impinge on reality.

It sounds familiar. It sounds preposterous. It is familiar. It is preposterous.The script also suffers from a few ghastly bouts of heavyhandedness, one troublingly inconsistent character (Waterston's), and a few tried-and-less-than-true laugh formulas. When a ridiculous question! I'm a mature, intelligent woman. Of course, I'm afraid of my mother." And one can almost feel the comic wheel clicking into places like the mechanism of a safe door.

But the wonder of "Lunch Hour" is how long Kerr, director Mike Nichols and the two stars manage to keep things really clicking. After a stodgy beginning -- and before the plot gets desperate in mid-Act-Two -- the comedy is lively and pleasing and coherent, with none of the grab-bag jokes that afflict so much stage comedy in the Neil Simon era. And when it comes time to get her play ready for the final curtain, Kerr is disarmingly out-front about the old paths upon which she has been treading.

With all five of her characters assembled onstage, she has leon, the manic landlord (played, very agreeably, by Max Wright) patiently explain just how this romantic dilemma would have been solved in the '50s, the '60s and the '70s. But "now that we've moved into the '80s," he says, "we've got to find a new way to solve this old, old problem."

And they do, more or less, sort of, after a fashion, find a new way.