That grand guru of the Baroque musical revival, Gustav Leonhardt, played an exquisite harpsichord recital at the Smithsonian's Baird Auditorium last night. The Dutch musician is one of the world's leading experts on the ins and outs of the Baroque. As harpsichordist, organist, conductor, musicologist and teacher he has influenced the new crop of Baroque musicians as much as any single person. Just one of his better known projects alone, the complete Telefunken recordings of the 200 odd Bach Cantatas (which he is doing with Harnoncourt), is a major landmark.

Leonhardt's keyboard style is exceptionally refined. There is none of the tortured bending and twisting of rhythm and lines favored by some Baroque players. The pulse is kept beautifully even. Leonhardt's sense of rhythm is subtle, allowing him to understate a work without its seeming dry. And he has a sophisticated sense of tonal nuance.

The program began with the unfamiliar. There were seven lovely vignettes--some of them descriptive--by the less celebrated Couperin, Armand/Louis. They ranged in mood from the jaunty "L'Arlequine" to the tender "L'Intre'pide," and were played with clarity and delicacy.

Then came five polonaises by one of the lesser Bachs, Wilhelm Friedmann, a son of Johann Sebastian. Wilhelm Friedmann lived to 1784, and one hears the approach of romanticism here. Do not expect them to sound like Chopin's polonaises, though.

The final work was the glorious J.S. Bach G minor Partita, the adaptation of the D minor violin partita, which closes with the sublime Chaconne. Leonhardt played it majestically.