"Groucho!" will fill up four weeks on the ford's theater calendar. It will fill up an hour and 45 minutes of your time. Otherwise, it fills on discernible need.

Lewis J. Stadlen, baptized into the role a decade ago in the flop musical "Minnie's Boys" (with Shelly Winters as the Marx Mother), is having a second go-round now with an evening of fragments from the movies, vaudeville routines and reminiscences of Groucho Marx.

You wouldn't confuse Stadlen with Groucho on a street corner, and there may even be impressionists out there who could best him in a Groucho voice contest, but he has the moves down cold. Not just the stooped-over walk - any fool could do that, any many have. Stadlen has mastered Groucho's bird-like sideways dart, too, and his way of collapsing cross-legged into a couch - or a wealthy woman's lap.

Stadlen and co-adaptor/director Denny Martin Flynn have rounded up many of the genuine treasures of Groucho's comic career:

Groucho singing the E. Y. Harburg/Harold Arlen "Lydia the Tatooed Lady" ("She has eyes that men adore so/and a torso even more so . . . When her muscles start relaxin'/Up the hill comes Andrew Jackson");

Groucho, in a poetic moment, telling Margaret Dumont, "Oh Emily, can't we meet some night under the moonlight? Just imagine, you and the moon. Wear a necktie so I'll know the difference."

Or, in the same vein, "I can see you now, bending over a hot stove. I can see you - but I can't see the stove." (The comic insult has come a long way since Groucho was in flower and it has all been downhill culminating in the phenomenon of Don Rickles being paid good money to call somebody a hockey puck.

The elderly Groucho being visited by the FBI after his ill-considered statement that "somebody ought to smack Richard Nixon on the nose with a banana." He hadn't meant it literally, he explained - "Anybody would be a fool to waste a good banana on Richard Nixon."

There's more, of course, but scarcely anything on this agenda will do more than jog the memory of a true Marx Brothers devotee. The selections include several songs - "Tit Willow," for one - are hardly quintessential Marx suggesting that the authors had copyright problems.

As for non-devotees, why should they have to settle for an imitation? "Duck Soup," "A Night at the Opera" and his other movies remain available for blissful viewing. So does "You Bet Your Life." From those sources and filmed Groucho interviews, a fair evening's entertainment could be constructed with Groucho himself as the star.

For all these shortcomings, it is easy to imagine this project, in less conscientious hands, having turned into a painful and pointless exercise. Stadlen has managed a sustained, convincing impersonation of someone whose most trivial gestures are imbeded in our comic consciousness, and that's a feat.

There is a woman in this one-man show, too. Nancy Evers' Margaret Dumont imitation may be a bit weak, but at the piano she does a splendid variation on Chico Marx, full of overwrought crescendoes and dainty finger dances.

GROUCHO! written by Lewis J. Stadlen and Denny Martin Flinn; lighting by Marc B. Weiss; costumes by Carol Oditz; produced by Richard Brodsky, Frankie Hewitt and Diana Enterprises.

With Lewis J. Stadlen and Nancy Evers, pianist.

At Ford's Theater through June 3. CAPTION: Picture 1, Nancy Evers and Lewis J. Stadlen in "Groucho!" by Harry Naltchayan - The Washington Post; Picture 2, Lewis J. Stadlen in "Groucho!"