Richard Bales received a long standing ovation before he raised his baton last night in the National Gallery, and another when he lowered it for the last time -- really the last time, because he will be retiring next month.

It was the 1,786th concert given in the museum's East Garden Court. Many of them have been conducted by Bales, and all have been given under his supervision as the museum's music director. "Concerts will be continued in the Autumn," promised the only line of type on the last page of the program, and Bales has reassured his friends that music at the National Gallery "will go on much as it has in the past." But it will not, cannot be quite the same; a successor will be announced, but nobody could be a replacement.

Musicians -- composers and performers -- were a substantial part of last night's overflow audience. They came to pay tribute not only to a colleague but to a particularly influential man whose influence was always exercised constructively.

In his 42 years at the National Gallery, Bales has used his primary role -- planning programs and selecting performers -- constantly, thoughtfully and systematically for the advancement of American music and musicians. It is safe to say that in that time no single person has done more to develop an audience for American composers, living and dead. He has also given a showcase to American performers -- notably young, relatively unknown musicians and those who specialize in worthwhile but unfamiliar music. Bales never had to worry about ticket sales for any of those 1,786 concerts, which were given free. He recognized the value of his freedom from the box office and used it creatively, sometimes in the service of music far from the essentially conservative idiom he favored as a composer.

Bales the composer occupied the first half of last night's program. It opened with his orchestral arrangements, dating from 1948, of two keyboard pieces by William Byrd -- Elizabethan music tailored to the taste of a modern mass audience in lush orchestral sound.

Considerably more substantial was his "Dirge for Two Veterans," based on a text by Walt Whitman, composed last year and orchestrated this year on a commission from the Alexandria Choral Society. The text is powerful, describing the joint funeral of a father and son who died in the same Civil War battle. And it cries out for music -- explicitly. Whitman virtually gives directions for a composer with repeated mentions of such effects as "full-keyed bugles . . . the great drums pounding and the small drums steady whirring."

The poet's cues are effective (though hardly a complete guide to orchestration) and Bales has heeded them while going beyond them in his use of the full orchestra and chorus. He writes for voices as proficiently as for brass and percussion; his harmonies and dynamic nuances powerfully reinforce the somber emotional flavors of the text. He is most effective when he is least spectacular -- for example, in the final quatrain given to a cappella voices after some dynamic instrumental passages and a declamation for a speaker (Douglas Olsen) over a percussion accompaniment.

The Alexandria Choral Society sang beautifully, though the acoustics took the edge off its precise diction. Bales ended the concert and his career with Schubert's Ninth Symphony. The acoustics, again, added flavor to the music, giving prominence to the bass line and sometimes blurring the treble in fast passages. But it was -- in the Bales tradition -- a noble, thoughtful, well-paced and highly effective performance.