WHOEVER ITS creators, whatever its budget, wherever it's performed, every play production is "experimental," though the term has come to suggest out-of-the-ordinary in out-of-the-way places.

Another term is Off-Off-Broadway, suggesting the Cino-La Mama-Cubiculo-Open Stage-Open Space genre in basements, restored buildings or rookeries where both new and old plays have humble but searching performances at modest prices and no salaries. As protest against established practices and outrageous costs, such groups exist even in cities that have no other theaters, filling the local theater vacuum with industry and hope.

There probably are a score of such enterprises in this area, serving ranging from the idealistic to the pretentious; but for integrity of purpose, noe is more distinguished than the New Playwrights' Theater at 1742 Church St. NW. This 76-year-old building, the theater's third home, was once a gymnasium for Holton-Arms School and, more recently, a dance studio.

This week, ranked as nonprofessionals under some professional guidance, members of NPT take off for New York, where, next month, experimental-minded Joseph Papp will present them in a production he admired in the old gym, "Hagar's Children."

This trek does not mean that anyone's idea of rainbow's end has been reached, though in old movies that would have been the finale. It simply means that the purposefulness of NPT's Harry Bagdasian and Paul Hildebrand Jr. has been recognized by another man who sees his work as "a life in the theater."

From the nights of the Washington Civic Theater, a good if now forgotten staple, I've been exposed to about every theater that's tried the Washington waters; but when I first heard of the American Society of Theater Arts, the progenitor of the NPT, I howled over the lofty title.

One aspect of the AMerican Society of Theater Arts quickened both mind and conscience; a scheme for producing new plays, the missing link between theater artists and theater audiences of our future.

So, on Nov. 16, 1972, I feretted out a basement room of an old house on 20th Street NW. There were 25 (or was it 24?) wooden chairs, a minuscule playing area and four new one-act plays. The borrowed lighting equipment was used knowingly and while the acting was modest, one was immediately aware that the whole was in the hands of those who had a clear idea of what they wanted.

Wary of first impressions, I nonetheless wrote about it enthusistically, kept returning and over time have developed an awed affection for ASTA. The aim continues the same: "To develop new dramatic voices without the need of "sell" reviews, and to test and trust the audiences. Consistently, other qualities have been added, including the enthusiasm of the town's theater people. Subsequently ASTA established larger headquarters at 612 12th St. NW. Later, the NPT group split and, through persuasive fiscal manipulations, achieved its own home a year ago in the old gym.

The plays have risen from dialogue exercises to full-length works and scripts pour in from all over. What one notes especially is their increasing assurance of construction. Usually they avoid chic, pointed formlessness in favor of a beginning, middle and end, although the styles may range from drama, historical impressionistic and intimate revue to sharp satire and twists on old themes.

From the beginning, dialogue has been more than competent but that gathering meatiness of the plays, their body structure, must stem from the guidance of two young men.

Founding-producing director Bagdasian, who went from theater major at the University of Maryland to managerial jobs in local professional theater, was the playwrights' unit head for ASTA when he voiced an enduring thought: "Playwrights need a stage, their own stage."

Artistic director Hildebrand, a William and Mary graduate, took his master's at Catholic University. Sometime actor and most credited of NPT directors, Hildebrand seems to have a sensitive ear for scripts and and eye for imaginative staging.

Another CU master's graduate is Robert Graham Small, who did technical work with Rockville's Street 70, Oiney, Folger and the American theaters. Raised to directorial assignments, his staging for "Hagar's Children" moved Papp to decide that if the play was to be staged at his New York Public Theater, no one could improve on Small's and his cast's work.

These are only the more obvious leaders of the NPT community. The ingenuity with which the open space is redesigned with which the open space is redesigned for each production, the technical invention with which the crew makes much from little and the feel of democratic effort have been on a rising curve under Bagadasian's searching leadership.

Created for playwrights, NPT also has progressed surprisingly in attracting good actors. At first one thought of the performers as earnest; one has grown to expect solid characterizations, a dividend for the four mayor productions in a season of numberless informal readings. This has built up active angels and workshop activists.

What, then, is lacking in our other "experimental" theaters?

One failing is a fuzzy idea of what a play is. It is not an acorn hidden in cotton. Some seem built on the notion that an audience is irrelevant to the superior musings of artists. Nor is a play the internal writhings of individuals drained of intelligible emotions in favor of wordy intellectualizing. Unintentionally, Freud has been a bad influence on good theater.

Another failing is ignorance of style, exemplified by a recent misguided effort to recreate the '30s of Noel Coward's "Design for Living." The players were encouraged were encouraged to think of inner conflicts, not the clipped, wordly Coward style. To hammer the "relevancy" of two men and a woman, one man was cast as black and the two men inserted sex by passionate kissing. This may be today, but it's not "Design for Living." Coward's style was to suggest the interior by outer decoration.

Thus, for its vision and measurable advances, its low-key attitude toward exploratory ventures and awareness of quality, Harry Bagdasian's trail-blazing group is at the top of our area's experimental theater scene. Not since Zelda Fichandler was leading Area Stage through its earliest years has the Washington theater produced so impressive, so quickening an institution.