Where the avant-garde flows into the mainstream. This season, theater and dance took center stage at America's leading showcase for what's new in the performing arts - the Next Wave festival at the Brooklyn Academy of Music. Today, David Sterritt does a wrap-up of the '87-88 edition, and Marcia Siegel comments on two of its boldest entries among the dance programs.

No one seriously doubts that the Brooklyn Academy of Music (BAM) is the leading American salon for forward-looking work in the performing arts. BAM's annual Next Wave festival has become not just a ``Who's Who'' but a ``Who's Going to Be Who'' of contemporary performance. A problem has dogged the Next Wave since it began, though. Mainstream audiences and critics praise it for introducing new and unexpected work. Yet some arts-world insiders question the boldness of BAM's choices. They say the most aesthetically radical and big-c Controversial performers rarely make their way into the spotlight there.

There's something to this argument.

BAM might answer, tritely but truly, that no festival can please everyone. BAM might also point out that its popularity rests partly on its high-wire walk between the innovative and the outrageous. Audiences know they'll be stimulated but not threatened by a typical Next Wave attraction. And that sells tickets.

I must agree, however, that tameness has marked much Next Wave programming. The festival has dodged the vital experimental-theater scene, for instance, where extraordinary developments have taken place in recent years. BAM has made impressive forays into opera, and dance is always a strong presence there. But pure theater, except for an occasional opus usually cooked up by Robert Wilson, has stayed largely in the wings - until this season, when theater rode to the rescue.

In the latest Next Wave festival, which ended recently, three plays as well as an opera made themselves heard. The things they said were often flawed, uneven, and more ambitious than successful. Yet in their bravery and freshness, these were the most invigorating messages of the season.

The most widely noted of all Next Wave events was Peter Brook's production of Jean-Claude Carri`ere's epic drama, ``The Mahabharata,'' a nine-hour spectacle based on an ancient Indian text. In spirit, if not in substance, it summed up everything the Next Wave ought to stand for: originality, skillful execution, and a willingness to abandon all conventions that don't serve the work in hand.

This said, it must be added that BAM spectators and reviewers greeted ``The Mahabharata'' with mixed emotions, and rightly so. Carri`ere's play is courageous in its scale, scope, and commitment. But rarely does it convey the deep human or spiritual insights that one might hope for, given its venerable text and its imposing subject - the nature of destruction and renewal in human consciousness, history, and religion.

Brook did a heroic job of bringing the play alive, but the weight and bulkiness of Carri`ere's text defeated him in the end. Brook has veered toward a radical simplicity in recent works for theater, opera, and film. His minimalism of technique is best suited to minimalist source material, however - not the sprawling maximalism of ``The Mahabharata.''

None of which detracts from BAM's great service in bringing this internationally acclaimed work to East Coast audiences - who responded by making it a winner at the box office, despite stiff prices and the play's great length. Audacious work like this is the proper stock in trade of the Next Wave, no matter how exhilarating or exasperating it turns out to be in the final analysis.

Two other bold experiences came from the teeming mind of Peter Sellars, one of today's most controversial stage directors. The first was his production of ``Zangezi,'' an experimental work subtitled ``A Supersaga in Twenty Planes,'' written by Soviet author Velimir Khlebnikov in the 1920s.

As with ``The Mahabharata,'' the text of ``Zangezi'' is a problem. As a poem, translated by Paul Schmidt, it's a windy and manic potpourri that anticipates James Joyce in its use of many styles to explore many ideas. Sounding like ``Beowulf'' one moment and Samuel Beckett the next, it's often too dense and diverse for its own good. Yet director Sellars transformed its unwieldy verbal energy into the foundation of a stunning theatrical event, marked by bravura performances and spare but concentrated visual events that drew from the text (including its excesses) unexpected meanings, overtones, and resonances.

I have never encountered a more fruitful use of ambiguity as a multiplier, not an evader, of meaning. Nor have I seen a more courageous effort to communicate on a wholly intuitive level, while maintaining a full commitment to verbal and intellectual values. Also impressive were the play's musical and multimedia effects, including a Jon Hassell score. Shortly before opening night, Sellars told me ``Zangezi'' is about ``the impossibility of grasping anything from the surface.''

Sellars's other BAM offering was ``Nixon in China,'' an opera by John Adams with an Alice Goodman libretto written in slant-rhyme couplets. In heroic-ironic terms, it deals with Richard Nixon's historic house call on Mao Tse-tung. Adams's music is most impressive in its artful merging of minimalist-style scales and arpeggios with vocal recitatives of surprising intensity. Sellars's staging was most obsessive when the music was at its most obsessive, too - as in the last scene, when the action took place around half a dozen beds lined across the stage. This and other episodes were memorable and relentless.

Further provocative stagecraft came from the Squat Theatre, a group of Hungarian 'emigr'es who've made history in recent years by developing their concept of ``storefront theater.''

BAM has no storefronts to offer, so it presented a more conventional Squat work called ```L' Train to Eldorado,'' about an inwardly tormented man who learns that mysterious moviemakers are shooting a documentary about his life and feelings. The characters and themes of the play seemed familiar, despite the unusual plot (in the end, our hero turns into a tree) and some offbeat staging. The play's imagery was powerful, though, showing that the Squat group has not lost its great visual imagination.

Turning to other arts, dance also made a good showing in the latest ``Next Wave'' lineup. The revelation of the season was ``Elena's Aria,'' by the Rosas Company of Anne Teresa de Keersmaeker, a Belgian choreographer who's been at BAM before. This work consisted of dancers getting into and out of chairs for a couple of hours, and while that sounds excruciating, Keersmaeker's inventiveness is so far-reaching that the result was transfixing.

The noted Compagnie Maguy Marin of France put on a two-part engagement, with its evocative ``Eden'' earning more cheers than the stifling ``Babel Babel.'' (Please see dance features, Page 21.)