"The Shadow Box," which last week won the Pulitzer prize for the best American play of 1976-77, is an eloquent and exuberant drama about the dying and those they are soon to leave.

Quite apart from the rich textures of Michael Cristofer's script and the uniformly expressive cast of director Gordon Davidson, the Pulitzer also recognizes the new American theater.

"The Shadow Box" was introduced a year ago by Davidson in the Mark Taper Forum in Los Angeles. Last fall Davidson again staged it at New Haven's Long Wharf Theater, and it is substantially this cast which opened at Broadway's Morosco Theater the night before the April 1 Pulitzer deadline.It is a product of American's regional theater.

This recognition has occured only once before in the past decade, with "The Great White Hope," which was introduced by Washington's Arena Stage the year before it won the '68-'69 Pulitzer. In these 10 years, three winners have come from Joseph Papp's Public Theater - "No Place to Be Somebody," in '70; "That Championship Season" in '73 and "The Chorus Line" in '76.

One was Orin Lehman's independent off-Broadway offering, Paul Zindell's "The Effect of Gamma Rays on Man-in-the-Moon Marigolds" of '70-'71. One was a Broadway production, Edward Albee's "Seascape" of '74-'75.

There were no Pulitzers for three seasons, '67-'68, '71-'72 and '73-'74.

Had "The Shadow Box" not come along, the judges would have chosen another regional creation to win this year's Pulitzer, "A Texas Trilogy." While there have been 12 years of no awards during the 61 years the prize has been offered, the Pulitzer regulations are very clear. The judges are not commanded to find a timeless work of art. They are simply asked what they consider to be the best play presented between April 1 and March 31 of a given year. This is what the Pulitzer is all about.

"The Shadow Box" came into New York with admiring respect from both Los Angeles and New Haven as well as encomiums from several New York critics, who proved rather less enthusiastic at its Broadway opening. In its first New York week, the box office was frail but the theater's best friend, word-of-mouth, has been helpful and now, perhaps, the Pulitzer will push it to the audience success it deserves.

Superficially, this sounds like a forbidding play to many who find life depressing enough anyway. But such is Cristofer's view of the joys of mere living that the play creates a sense of celebrating life. The stings of humor and irony quicken what might have been lugubrious sentimentality.

In the play, there are three cottages on the grounds of a large California hospital. When the wife and son of the occupant of Cottage One arrive, we are uncertain what is troubling her. Have they separated? Is this an attempt at reunion? Whatever, there is something the wife avoids talking about, and she finds endless excuses to keep from entering the cottage.

The patient in the second cottage seems to be an intellectual cared for by a young man. They are expecting the older man's divorced wife, and after she arrives, we realize that the wife, who flaunts jewels she has picked up as "your average tramp," is not needed and that the youth is the husband's lover.

The third patient is a testy old woman in a wheelchair who seems to be waiting the arrival of a loved, glamorous daughter, a lively contrast, we assume, to the plain, older daughter who looks after her. In time we will learn that the beloved daughter has long been dead and that the scorned one has been falking latters from her dead sister who seems to be so busy traveling and enjoying life. To reveal this would ease the scored sister's present life but deprive the old woman of her desire to live for reunion with her other daughter.

In the first situation, the wife's fear of death deprives her of enjoyment of life. In the second, however fake the intellectual may be, he has enriched the life of a youth who previously had been a drifting, ignorant hustler. In the third situation, the mere idea of life's enjoyments, which she cannot now share, feeds the mother's life-hunger.

What Cristofer is really discussing, then, is the estasy of living, the carelessness with which we take the gift of life and the overpowering importance of the "now." There is a beautiful sense of formal ritual that Davidson devises for the final scene.All these people, who do not know the people in the other cottages, turn to the audience and assert "this moment."

Through Cristofer's restrained impartiality and Davidson's sensitive staging, the characters simultaneously achieve universality and individuality, roles with which actor-playwright, Cristofer rewards fellow actors. Joyce Ebert's troubled, fussing wife is a memorable drawing of muted heartback opposite Simon Oakland's clam but desperate husband. As their boy, Vincent Stewart strums his guitar with Davidson letting us see him as the 16-year-old probably feels, removed from the two who created him.

Patricia Elliott brings glittery precision to the intellectual's vivid wife, and Mandy Patinkin's explosive but controlled Mark shows the growth this association has given to the hustler, an immensely provocative performance. Laurence Luckinbill's intellectual suggests terror through his armor of words.

Geraldine Fitzgerald's old woman is that actress finer than she's been in years; her scenes with Josef Sommer, as the hospital interviewer, indicate the woman she once was. As the scorned daughter, Rose Gregario's superbly understated revelation scene is the play's finest.

While some of these exceptionally fine players have been on New York's stages before, for the most part they are regulars of the regional theaters from Olney and New England to the Midwest and California.

At Catholic University, where "that Championship Season's" playwright, Jason Miller, also was a student, author Cristofer was known as Michael Procaccino, the name he used as an actor with Arena Stage, the Garrick Players, Wayside Theater, Olney and the Washington Theater Club.

It could not be said during this first 50 years, but now the Pulitzer is recognizing that our theater is becoming national and is not limitedto New York.