Trevor Pinnock conducted his small but pungent orchestra, the English Concert, sitting at his harpsichord last night in the Kennedy Center Concert Hall. He was not merely the conductor but very much a member of the 20-piece ensemble he founded 15 years ago.

Much of the time his instrument, filling in the harmonies, setting tempos and providing a focus for the orchestral members, was no more audible to the audience than a baton would have been. But it clearly enhanced the stylishness of the group's performance on old instruments, and when it absolutely needed to be heard -- when Pinnock played the solo in Haydn's Keyboard Concerto in D -- it came through clearly and elegantly. When it served as a continuo instrument, an occasional chord found its way to the back of the auditorium.

The program gave a fairly extensive sample of 18th-century orchestral styles, from a delightful violin concerto composed in 1719 by Georg Philipp Telemann to theHaydn concerto, dating from 1782. The other two works were symphonies from the 1760s.

Johann Christian Bach's Symphony No. 6 in G minor is an energetic, gracefully moody little work from which Mozart (just entering his teens when it was published) may have learned something. Haydn's 59th Symphony in A (nicknamed the "Fire" symphony, possibly in tribute to its ardent style) is of roughly the same vintage as the J.C. Bach work but somewhat more imaginative and technically advanced. Among its many felicities was a literally breathtaking dialogue in the finale between the oboes and the high-pitched valveless horns. It was brilliantly effective, though the horns had more trouble than the oboes staying on pitch.

In the Telemann concerto, the orchestra's concertmaster, Simon Standage, soloed as effectively as Pinnock did in the Haydn.