NOT SO LONG ago you could barely get an audience for plays which hadn't been trumpeted from New York.

Now, across the country, new plays are in: some companies devote whole slots of time to a "New Play Festival," "Playworks" or "Playmarkets", making it possible for addicts to see as many as six new plays in three days, even more by adding morning "Workshop" novelties.

The vogue, a welcome one, is decidedly strong in Washington. A few Sundays ago, when six active playwrights were interviewed here, none said that life was a breeze -- but all at least have had the satisfaction of hearing their plays before audiences.

The current American College The-ater Festival includes four new works in the Kennedy Center's Terrace Theater, with UCLA's "Endangered Species." by Richard Larson, to be given three performances this Thursday and Friday.

Hampered by its ill-suilted temporary quarters in the American Theater, the New Playwrights' Theater of Washing ton nonetheless raised some $17,000 during its annual Dramathon, which the D.C. City Council saluted with a day in its honor.

Arena Stage's "In the Process" series consistently fills its Old Vat Room and from its main stages plays have been introduced which have had further life elsewhere.

The Folger Theater Groups's "Whose Life Is It. Anyway?" has been followed by a New York production of Brian Clark's London success, and the Folger's just-announced liaison with the Kennedy Center assures expansion for its new play format.

This movement has several interesting aspects:

1) It is genuinely national. While some theater centers, notably Chicago and Roston, once did produce new plays, new play-making now is happening literally everywhere.

2) Most of the plays will have no further life than their original productions. Some, in fact, are sure to make shuddering audiences ask why they were produced at all. There is nothing novel about such reactions because playwriting is a deceptive craft' What looks inevitable is complex; what seems to work on paper doesn't before an audience. Far more plays will fail than succeed.

3) The current crop of plays, usually by writers in their 20s, represent a heartening switch from the preceeding generation. In place of introspective meanderings, usually of a doleful hopelessness, the younger writers are thinking outside of themselves and their eyes are looking upward. After a seemingly endless oh-the-pain-of-it-all period and despite subject matter that may seem jolting to their elders, the young are finding again the bouyancy that once seemed a national characteristic.

In the forefront of these national marathons is the Actors Theater of Louisville, which held its third annual New Play Festival during the February blizzard.

The schedule was arranged so that six premieres might be rehearsed, introduced gradually and then, for two final weekends, all could be seen within three days. Three other plays would have morning "Workshop" performances, with books in hand if need be.

What a flood of assorted professionals -- producers, agents, play wrights, technicians and critics -- then dropped from the Kentucky skies!From California. Texas, Chicago, New York, Washington, Canada and England, the band of play-searchers moved into the marbel lobby of the old Bank of Lousivlle, built in 1837, and now the entry to ATL's two theaters. Why had they come?

Two years before producing, director Jon Jory, who'd heard about an earlier production in a tiny California theater, had come up with the next year's Pulitzer winner, "The Gin Game," which the Cronyns now are playing so brilliantly at the Eisenhower. The second year he'd found a Louisville playwright, Marsha Norman, whose "Getting Out" also had gone on to further productions.

Son of actor Victor (who was among those seeing new plays in the Victor Jory Theater). Jon Jory is in his 10th Louisville season, where his company has become one of the most reliable in America. Founder of New Haven's Long What Theater, Jory created Louisville's Great American Play Contests, which netted so many worthy entries he expanded them into an annual festival. He also has been guest director at Arena Stage. San Francisco's American Conservatory Theater and the Oregon Shakespeare Festival. This spring he became the first producer to be a two-time winner of the Margo Jones Award for introducing new plays, first at New Haven, now in Louisville.

While this third year did not come up with "The Gin Game's" counterpart, there were decided rewards, two playwrights in particular. James McLure and Beth Henley, and a novel Parade titled "Holidays." short pieces by 10 noted writers. It seemed to me striking that both McLure and Henley earn their havings as actors, as do several other of this year's new playwrights. Their dialogue could teach the professors.

McLure is a striking writer, terse, jolting, dry and humorous. In "Lone Star" we meet two Texas brothers. While the brash one has been in Vietnam, the quieter younger had an affair with his brother's wife and, just as importantly it seems, smashed up his T-Birtd. A hangeron is dim witness to implied truths: Leo Burmester, Patrick Tovatt and Peter Bartlett played the rich roles perfectly.

McLure's more serious but just as dry "Pvt. Wars" had a workshop performance and fortified one's confidence that he is a coming writer of prime rank. I'm looking forward to seeing how he handles more than three characters in longer plays.

Henley's fuil-length "Crimes of the Heart" is about three sisters who can't seem to leave Hazlehurst, miss. Like McLure, Henley has a player's fine ear for solid dialogue. She also has humorous quirky twists, perhaps suggested by one of the girls' reason for shooting her husband: "I never liked his looks."

While its action gets bumpy, "Crimes of the Heart" should one day find wide audiences for its immensely likable characters and the play's essential bouyancy.

"Holidays" is 10 shortplays plays by Rex Aranna, Tom Guare, Obner Hatley, Israel Horowitz, Preston Jones, Marsha Norman, Megan Terry, Douglas Turner Ward, and Lanford Witson (almost a rundown of today's most active playwrites). The Fourth of Julyinspired several of the vignettes and some could be (line illegable) is "The Great Labor Day Classic" by Horowitz. (line illegible) players become marathon runners. (word illegible) says "New Year's" probably is the most impressive, an arresting study of a prostitute who finds her date for the night is Death. CAPTION: Picture 1, Francis Foster, right, in "Nevis Mountain Dew": Not the mammy type."; Picture 2, On the national scene -- "Crimes of the Heart" at the Actors Theater in Louisville,; Picture 3, "Endangered Species" from the American College Theater Festival.