There was only one composition on last night's National Symphony program, but all of music's many moods and voices were encompassed in a performance that lasted nearly two hours. The program was Mahler's Third Symphony -- a deep, complex celebration of nature, of man's place in it and of his aspirations to eternity and infinity.

It is one of the great monuments of the human imagination -- a work of art comparable, in its way, to "King Lear" or the cathedral at Chartres. It is music greater than any one performance can possibly show. But last night, Mstislav Rostropovich, the National Symphony, contralto Maureen Forrester, the women of the Oratorio Society and members of the Washington Archdiocesan Children's Chorus explored its depth and variety of statements with enormous effect. A long standing ovation was the performers' just reward. Special applause was given to Milton Stevens and Adel Sanchez for their solos on the trombone and posthorn.

Mahler's Third Symphony is protean, ever-changing and frequently ambivalent in its moods and styles. A climax of crushing power gives way almost immediately to a lilting little dance, as light and uncomplicated as a breeze on a summer morning. The sound's textures vary in an instant from the full-throated roar of an augmented orchestra (eight horns, four trombones, five percussionists, two harps) to a solo instrument cushioned on a muted whisper of orchestral background.

After three movements of purely instrumental sound yearning for the special clarity and power of the human voice, the symphony breaks out into words. But for its final statement it returns to pure sound, less explicit but enormously more suggestive: a long, ecstatic adagio, quiet for most of its length but rising, finally, to one of the most satisfying climaxes in music.

The performance, too, was satisfying if not perfect. The broad outlines of the music, its progression through the varied moods of six movements, were seen and presented as a unified whole by Rostropovich. Many details were etched with superb focus and clarity, and he ventured beyond the deadpan modern style into a lightly Viennese musical accent. There were a few opening-night problems not likely to be repeated: little lapses of ensemble, a blurred entry by the second violins at the beginning of the second movement, occasional intonation that was not dead center.

Not exactly a problem but a passage short of ultimate perfection was the early part of the final movement. It is marked "Slow, Peaceful, With Feeling," and the performance was all of these. It is infinitely more difficult to achieve intensity when playing slowly and quietly than when playing loud and fast. Rostropovich and the NSO are masters of the latter art, but a few orchestras and conductors can obtain a greater level of intensity in the slow, quiet opening of the last movement than was heard last night. The NSO has grown enormously under its current music director, but it still has room for more growth.

Those who heard Maureen Forrester singing Mahler 20 years ago will note the inevitable ravages of time in her voice: some loss of power, less richness of tone and less ease of legato in long phrases. But it remains a great voice and one used with extraordinary artistry by a singer whose authority in Mahler's music is second to none.

Both choruses were trained by Robert Shafer, and the quality of his preparation was evident in their relatively brief but pungent contribution to the symphony.