Harpsichordist Judith Norell brought fluency and verve to a program of Bach concertos at the Washington Cathedral last night, thereby mocking the old adage that those who can't, teach.

Conducting from the keyboard, Norell, who occasionally gives lessons to William F. Buckley Jr., led violinist William Steck and a mini-chamber orchestra--consisting of a second fiddle, a viola, cello and bass--through the Fifth Brandenburg and three other popular works.

Braving high humidity before a large crowd in the cavernous cathedral, the performers managed, in the Concerto in G minor for Harpsichord, B.W.V. 1058, to strike a careful dynamic balance, with the keyboard always prominent, while blending in tight ensemble. They did the same in the Concerto in F Major, B.W.V. 1057, for which Thomas Perazzoli and Sara Stern on flute contributed delicate phrasing and a lyrical tone.

Norell, who leaped into her few cadenzas with obvious relish, is a controlled but not necessarily restrained performer; her playing is marked by ease of articulation, clean attacks and a sense of near-perpetual motion.

This last quality was perhaps overworked, especially in the adagio of the Concerto in D minor, B.W.V. 1052, which sounded barely more leisurely than the first-movement allegro, and might have profited from some expressive rubato. But that's a quibble about lively playing, as in the Brandenburg, which came together smoothly with the dance-like finale sporting a rustic charm.