The audience was still applauding after midnight last night in the Kennedy Center Concert Hall for the epic preliminary to a major event in the history of the National Symphony orchestra. The concert, given only once, was in effect the final rehearsal for a recording of Mussorgsky's "Boris Godunov," which will begin tomorrow in the Concert Hall and will undoubtedly be the most ambitious recording in the orchestra's history.

The performance, begun at 7 p.m., had been designed to include all the music Mussorgsky composed for this opera -- a tricky assignment, since the 1872 revision recycled some of the 1869 text in new contexts. There had been a standing-room audience at the beginning, and there were some empty seats by the time the soft, despairing notes of the Simpleton's lament faded into silence nearly five hours later. But despite some longueurs and occasional nai vete', this performance left no doubt that "Boris Godunov" is a great work of art and that it had been treated, on the whole, as it deserves.

The single concert performance followed 10 days of rehearsal for a recording that will circulate not only in pure audio formats but in the sound track for a movie and, eventually, a home video recording.

The cast of "Boris" is enormous, and the evening brought a few surprises as well as some expected excellence from usually fine performers such as Paul Plishka (Pimen) and Nicolai Gedda (the Simpleton). The use of artists of this caliber in such secondary roles is an index of the kind of quality being sought by the NSO -- and Erato, its recording company for the project -- in this "Boris." Other internationally known names included Kenneth Riegel in the role of Shuisky, who is more of a villain in this opera than Boris, though less spectacularly.

Ruggero Raimondi, has a filmed Escamillo and "Don Giovanni" already to his credit, but for many in last night's audience, his previous film appearances left unanswered the question whether he has the weight, musically and dramatically, for Boris. But by the end of the evening, after a death scene in which he indulged in none of the traditional rug-chewing but made a rich, finely nuanced dramatic impact, there was no further question. This is a Boris of great substance, gifted with a voice of depth and color to match the role. He sang with the kind of fine nuance required for video recording.

Some of the best singing was heard from singers unfamiliar or only occasionally heard in Washington. Vyacheslav Polozov, as the False Dmitri, grew into his music as the evening progressed and ended impressively. Romuald Tesarowicz (Varlaam) sounds like a major talent -- a first-class comic with a rich, mahogany-toned voice. Mira Zakai has vocal and acting talents that obviously go far beyond the limits of the role of the nurse. Catherine Dubosc was excellent in the role of Boris' daughter Xenia and Mathew Adam Fish had a certain kind of excellence as the Tsar's son, Fyodor.

More often than not, recordings of this opera use a woman's voice for the heir apparent -- but it is harder to do this when there will also be a visual dimension. Fish, a 13-year-old who already has a remarkable amount of musical and theatrical experience, sounds like a boy treble -- sometimes uneven in dynamics and a bit stiff in phrasing and diction. But presumably this was the sound Mussorgsky expected when he wrote the role, and the use of an adult woman soprano would be odd in a performance that uses his original orchestration.

There was one serious departure from his concept -- the use of a single soprano, Galina Vishnevskaya, in two very different roles: the Polish princess Marina in Act 3 and the lusty hostess of the inn in Act 1, Scene 2. Vishnevskaya, one of the unique voices of the century, retired from the operatic stage (though not from recitals or recording) a few years ago, and last night's performance made it clear that she had acted prudently. Much of her voice still remains; she provided some thrilling moments and her acting was revelatory as Marina, richly comic as the hostess. But her vocal control, in live performance, no longer meets the standards she established in her roles. In a recording studio, with editing and retakes, it should be possible to put together good interpretations of both these roles. But on stage, she did not always sound like the great Vishnevskaya. The phrasing was uneven, the tone often but not always firm and rounded, the dynamics quite variable and the balance of voice with orchestra and/or chorus sometimes problematic.

Not at all problematic were the contributions of the National Symphony Orchestra, in top form, or the three choruses who represented the Russian people -- in many ways, the real heroes of the opera and the beneficiaries of some of its greatest music. The Choral Arts Society and the Oratorio Society were seated not on the stage but in the front rows of the auditorium. Their sound did not have to cut through that of the orchestra to reach the audience, and the sound was striking. Also excellent were the Russian diction and the dramatic interpretation. The Chevy Chase Elementary School Chorus, seated at the side of the stage, had less sonic impact most of the time but will probably emerge well in the recording.

Rostropovich shows a flair for drama in just about anything he conducts -- and in this music it was not only fully exercised but highly appropriate. The evening was not flawless but it showed the ingredients of what could be a first-class recording and film.