In their mere four seasons together, the members of the Smithson Quartet have made themselves something of an indispensable factor in the musical life of Washington. Playing on priceless stringed instruments from the collection of the Smithsonian Institution, this group specializes in works of the Classic and early Romantic periods. Last night at the National Museum of American History they presented a program of three late 18th-century works. The last was a somewhat small-scale reading of Beethoven's Op. 18, No. 1 quartet. The famous Adagio, which was inspired by the tomb scene in "Romeo and Juliet," needed more color and a broader tempo.

But the first half of the program was treasurable. It consisted of two highly atypical late Mozart compositions, both dating from 1787: the famous "Eine kleine Nachtmusik" and the infamous "Ein musikalischer Spass" ("A Musical Joke"). What makes "Nachtmusik" atypical is its extraordinary simplicity. "Nachtmusik," with its symmetrical tunes, radically simple harmonies and dominating first violin, might have been written by Mozart at 18 rather than 31. Only its sweet wisdom and mellowness give it away as Mozart at his ripest, all artifice stripped away.

"A Musical Joke," on the other hand, is an almost cruel satire on composers who depend entirely on artifice but haven't even the basic musical skills to get them from one formula to the next without disaster. After a silken rendition of "Nachtmusik" the Smithson Quartet tore into "A Musical Joke" with something like maniacal glee. The first movement gave only a hint of what was to follow, with its banal themes and slightly sour harmonies. The belly laughs came in the minuet, full of empty climaxes, horns yowling in the wrong keys, hideous voice leading. In the trio, the first violin indulges in scales out of some Mad Hatter's exercise book.

The Smithsons played all this in strictly deadpan style, allowing the audience to provide the smiles and laughter, which they did in generous measure. The whole performance suggested that PDQ Bach's real ancestor was W.A. Mozart. A final word of praise is due the two unnatural natural horn players, R.J. Kelley and Lowell Greer.