Many people arrange for special leave when their birthdays fall on work days, but not Richard Bales. For the music director and conductor of the National Gallery Orchestra, the timing of his 70th birthday could not have been more fortunate. Bales spent last night where he spends most Sunday nights: at the podium in the museum's East Garden Court.

At the request of gallery officials, the program consisted entirely of Bales' own work, representing nearly every decade of the composer's output. Opening the concert were "Music for Strings" (1940) and "Theme and Variations" (1944). They introduced Bales as a smooth Neo-Romantic who demonstrates the depth of sound found in late 19th-century symphonists. Bales conducted in his typically unflappable fashion. There were a few lapses in upper string intonation in his Theme and Variations, but mostly the music was clean and lucid.

Contralto Beverly Benso joined the orchestra for "A Set of Jade," a work performed just last week in its piano version at the Phillips Collection. This beautiful, elegantly orchestrated song cycle subtly draws on oriental motifs. Benso's dark, mellow voice brought drama to the ancient Chinese texts, although at times she failed to sing above the orchestra.

The "Suite for Orchestra" (1984), subtitled "The Spirit of Engineering," and National Gallery Suite No. 3 (1957) revealed an adventuresome Bales susceptible, respectively, to the sparkle and energy of Richard Strauss and to purely "American" descriptive devices. Except for some poor brass attacks, the group played like clockwork, delivering highly effective performances of this genuinely congenial music. The concertmaster initiated the surprise encore of -- what else -- "Happy Birthday."