There are times when it is hard to believe that Gary Karr really is playing the double bass, that instrument often labeled, with some justice, a doghouse, a bullfrog, a huge, unwieldy monster.

Yet last night on the stage of the Kennedy Center's Terrace Theater Karr stood with his arms around his beloved 1611 Amati bass and made exquisite music. And he often made it at speeds that seemed staggering when you realize how much distance his fingers must cover and how much pressure he must exert on the big strings, compared with his colleagues who play the higher-voiced cellos, violas and violins.

The finale of Aaron Copland's Violin Sonata, which Karr transcribed with the composer's blessing, is a blistering affair even for violinists. Written just before "Appalachian Spring," it holds some of Copland's most appealing writing. Karr made it sound easy, almost, but he is too honest ever to try and fool you into thinking something is not hard when it is, in fact, very difficult. You can see him work as he plays.

Karr is a superb, interpretive artist who happens to make music through the medium of an instrument few have ever conquered to the point of achieving solo careers. Last night he recalled two of his three great predecessors, Bottesini and Koussevitzky. He played the latter's Andante and Cantabile and Valse Miniature, which are the epitome of the morceaux de salon that used to delight the inhabitants of Parisian salons. From Bottesini, who was a conductor and composer as well as a phenomenal bassist, he knocked the spots off a razzle-dazzler of a Reverie and Tarantella.

The evening held much besides big show pieces. There was a lovely account of Schubert's Arpeggione Sonata, which works as well on the double bass as on the more usual cello. One of the famous Eccles sonatas, at the beginning of the evening, gave a clear idea of what lay ahead.

Harmon Lewis is the piano half of the Karr-Lewis duo. His playing, notably in Copland and in the sonata Paul Hindemith wrote for bass and piano, was outstanding. The Hindemith is a rich treasure that we are prevented from hearing often simply because there is only one Gary Karr.

At the end of the recital the Terrace Theater was filled with the kind of ovation that has become routine in that theater.