In terms of activity, Off Broadway has been far outpacing Broadway this season. The situation is not unprecedented. The cause is partly economic. Entertainments that might once have risked Broadway can be mounted for much less money in the theater's outskirts. Off Broadway has become the natural habitat for the offbeat, the small musical, the play with perhaps a special appeal, the lesser-scaled, and the occasionally experimental.
There is sometimes the hope that an Off Broadway success can be transferred to the main stem. Currently such transfers include ''A Chorus Line,'' ''Crimes of the Heart,'' ''Joseph and the Amazing Technicolor Dreamcoat,'' ''Pump Boys and Dinettes,'' and ''Torch Song Trilogy.''
Recent Off Broadway arrivals have demonstrated again that, although we constant playgoers may not always travel in Keats's ''realms of gold,'' we do get around. In the course of several such outings, I have visited the worlds of sports, 17th-century Mexico, Berlin in the 1920s, and the state of Tennessee (Williams, that is). Williams play
a/k/a Tennessee, which closed after a very brief run at the South Street Theater, took a leisurely, two-hour excursion through Williams country - a terrain of fragile, vulnerable, and frequently troubled people. The retrospective collage, subtitled ''Facts and Fictions of Thomas Lanier Williams, '' consisted of play excerpts, poetry, and reminiscences. The collaboration honored the playwright and did credit to his interpreters.
Maxim Mazumdar, who devised the program, respectfully selected the principal subjects of Williams portraiture, from Amanda Wingfield of ''The Glass Menagerie'' and Blanche DuBois of ''A Streetcar Named Desire'' to Alexandra Del Lago of ''Sweet Bird of Youth.'' The anthology also took due note of the playwright's comic spirit - his gift for irony (not forgetting self-mockery), and the humorous observation of such minor pieces as ''The Case of the Crushed Petunias'' and ''A Perfect Analysis Given by a Parrot.''
In the casual biographical fragments, Mr. Mazumdar recorded Williams's early struggles to establish himself as a writer, followed by his sudden success with ''The Glass Menagerie'' in 1945 (he later referred to ''the catastrophe of success''). The script touched on the playwright's bouts with Hollywood, drug and alcohol addiction, his prolonged mental breakdown, his homosexuality, and the wounding critical rejection that followed so much acclaim.
With Mr. Mazumdar serving mostly as an unassuming but authoritative stand-in for Williams, fellow players Carrie Nye and J. T. Walsh populated the stage of the South Street Theater with numerous characters from the author's plays, stories, and reminiscences. The performance, directed by Albert Takazauckas, displayed the kind of dedication that best serves the characteristic Williams strengths - lyricism, wry humor, melancholy, and compassion. Peter Harvey's neat but unpretentious scenery, lighted by Mal Sturchio, contributed to this appreciative guided tour through the states and stages of Tennessee Williams. The Anastasia mystery
Multiple-role playing in quite another zone of time and place makes its own histrionic demands in I Am Who I Am, at the Perry Street Theater in Greenwich Village. British dramatist Royce Ryton is revisiting some of the scenes of a mystery that has intrigued historians, fictionists, and the general public since the 1920s. Mr. Ryton is reconsidering the case of one of the many women who claimed to be Princess Anastasia, asserting that she had survived the execution of the Romanov royal family of Russia by the communists in 1917.
The Anastasia of the 1954 Guy Bolton adaptation of Marcelle Maurette's romantic drama satisfied the Dowager Empress of her legitimacy, only to vanish. Mr. Ryton takes a different tack. The Dowager Empress and several other royal relatives (played variously and adroitly by Lucille Patton and Jeff Abbott) concur in recognizing Anastasia until they hear about the 22 million rubles in gold the Czar supposedly deposited in British banks. ''I Am Who I Am'' ends with the mystery still unsolved.
Mr. Ryton attributed their retractions to greed. On Anastasia's visit to New York, a pair of unscrupulous opportunists attempt to form a syndicate to pursue her claim through the courts. (According to the book ''Nicholas and Alexandra, '' by Robert K. Massie, the Grand Duchess Olga, who had been closer to her niece Anastasia than any other Romanov survivor, rejected all pretenders. Massie quotes her as saying: ''My telling the truth does not help in the least, because the public simply wants to believe the mystery.'')
Rather than solving it, Mr. Ryton assembles numerous small pieces of a jigsaw puzzle into a plausible and reasonably absorbing suspense play. His Anastasia claimant, a Mrs. Manahan (Leslie Lyles), so convinces a kindly German police inspector (Nick Stannard) of her genuineness that her case becomes his cause.
Under Jaemes Esterly's direction, the performance carries the kind of make-believe conviction due a well polished piece of stagecraft. Neil Jacob's clever, all-black scenery, lighted by Frances Aronson, facilitates the plot's unfoldment in a succession of brief scenes. The Esterly-Janet Eiger period costumes contribute richly to the flavor of the Ryton speculations. 17th-century prodigy
A woman's ordeal of a far more significant kind and consequence occupies The Price of Genius, at the Lambs Theater on West 44th Street. Betty Neustat's costume drama concerns the achievements and subsequent ordeal of Sister Juana Ines de la Cruz, a 17th-century poet, dramatist, and scholar who has been called ''the genius of Mexico.''
Miss Neustat chronicles how this young prodigy won an unprecedented place in Mexico City's male-dominated society. The gifted provincial then rejected an advantageous marriage in preference for the convent life she thought would free her for creative and intellectual pursuits. In the end, her brilliance was no match for the machinations of a local bishop (Bob Cooper) who envied Sister Juana's worldwide renown and determined to end her illustrious career. Clerical oppression stifled her academic freedom.
''The Price of Genius'' is a literate, impressively theatrical work. Sister Juana - beautifully portrayed by Patrizia Norcia - is a woman who wins her way by grace, humor, and charming forthrightness as much as by intellectual acumen. Principals in the intelligent performance, directed by Sande Shurin, include Alfred Karl, Rae Kraus, Sterling Swann, Jeremy Brooks, and Patricia Mertens. The handsome production was designed by David Potts (scenery), Richard Nelson (lighting), and Patricia Adshead (costumes). Domestic side of pro sports
Finally, on to the sports sector and Baseball Wives, at the Harold Clurman Theater. Grubb Graebner's knowledgeable-sounding comedy deals with the day-to-day experiences and play-by-play reactions of three such wives in the course of a pennant-winning season for the Houston Astros. The ballplaying spouses never appear on stage.
Doris (Carol Teitel), a veteran of many seasons, is caught up in her manager-husband's dream of a long-sought pennant. The philosophical Janelle (Marcella Lowery) is married to a philandering black big-league champion who has seen his greatest days. Scholarly Becky (Lynn Goodwin), the wife of a promising rookie, discovers to her chagrin that marriage to a player means being wedded to the game.
The action moves back and forth from the grandstand seats reserved for these voluble wives to their respective apartments - all neatly arranged by designer John Falabella. Away from the field, the women cope as best they can with the strain and stress of being married to professional athletes. They argue, gossip, share confidences, seek escape in alcohol or marijuana and - when the chips are down - support each other.
Under Gloria Maddox's direction, the playing is crisp, well paced, and perceptive. An evening spent with ''Baseball Wives'' leaves one with a certain new sympathy and respect for the women behind the men who play the great American national game.