After a superb concert of his music last night, William Schuman relaxed and chatted with his audience in the Terrace Theater. The conversation, like his music, was free-flowing, easy and natural. Schuman is almost a national monument in American music, not only as a composer but as the founding president of Lincoln Center and, for 17 years before that, president of the Juilliard School.

He is also as unpretentious (musically and in person) as any composer alive. Rather than his achievements, he talked about his youth as a pop songwriter: "I composed one song with Frank Loesser . . . it was the only flop he ever wrote."

Or about the plight of the composer in our time: "There's a lot to complain about, but it's not a very useful exercise . . . Nothing tells you that you have to make a living out of music, and nothing tells you that if you write music it has to be performed all the time. That's not your battle; your battle is to fight an empty page successfully."

Schuman has composed as many symphonies as Beethoven, and is probably best known for the third of them, for his brilliant orchestration of the organ "Variations on 'America' " by Charles Ives, and for his opera based on "Casey at the Bat," which is more often mentioned than performed. But last night's program was devoted to relatively unfamiliar chamber and choral music.

Before intermission, there were two lengthy, contemplative, beautifully composed works. "Night Journey," a tense, dramatic piece for 15 instruments written as a ballet for Martha Graham, was exquisitely performed by the Peabody Contemporary Music Ensemble, Frederik Prausnitz conducting. "In Sweet Music," a setting of a Shakespeare song, is preceded by a long, wordless meditation on its melody. Last night, soprano Rosalind Rees, flutist Carol Wincenc, violist Donald McInnes and harpist Susan Jolles made one wish the melody would never end.

In the second half, Paul Traver and his superb, 40-voice chamber ensemble from the University of Maryland Chorus sang a selection of his choral works with their customary verbal clarity and rich, balanced tone. Highlights included the wittily deadpan "Five Rounds on Famous Words," "Perceptions" (like "Night Journey," a Washington premiere) with richly emotive texts from Walt Whitman and, for a powerful conclusion, a terse, energetic "Te Deum."

The Kennedy Center's American Composers Series, which concluded its second season with this program, is one of the most innovative and fascinating events in the rich musical life of this city. The choice of Schuman for the final presentation, with the superb craftsmanship of his music and the easygoing warmth of his personality, brought it to a fine conclusion on a tonic major chord after some earlier dissonances.