Keith Jarrett performs with Handel & Haydn

Artistic director Christopher Hogwood has been stirring some adventurous programming into the Handel & Haydn Society mix of baroque and classical music since his arrival a year-and-a-half ago. As he told a sellout Symphony Hall audience Friday, he wants to break down traditional barriers between ``the popular and the presumably unpopular, to experiment with reactions - yours and ours, and to get over predictability.'' His new ingredient in the Friday and Sunday H&H concerts was jazz pianist Keith Jarrett, who is no stranger to the classics. In addition to his jazz trio work with bassist Gary Peacock and drummer Jack DeJohnette and his improvisational solo concerts and recordings, Jarrett has been performing music by serious composers, both classical and modern, for about five years now.

With Hogwood conducting, Jarrett gave a rather tame reading of Mozart's Concerto No. 21 in C (K. 467) from a score that, as if to underline Hogwood's emphasis on the unexpected, kept closing. Jarrett's technical skill was usually equal to the demands of the piece, but he seemed to be reaching for whatever ballad-like elements it had to offer, without summoning the inner fire that keeps Mozart's lyricism from turning limp and banal; the languid Andante movement seemed especially slack. As a result, the performance failed, for this listener at least, to sustain the necessary tension and energy.

Among the most effective passages were the two solo cadenzas, where, unexpectedly, this skilled improvisationalist used the Paul Badura-Skoda versions rather than his own but also where, happily, his style seemed less at odds with Mozart's.

Fortunately Jarrett's Piano Improvisation was far more sstisfying. With an aura of total concentration, he created a compelling and resonant six-section piece melding Bach and Basin Street. Inventive melodically, harmonically, and rhythmically, it paid homage simultaneously to baroque counterpoint, classical style, and Jarrett's own 20th-century roots. This deeply felt outpouring won the evening's most enthusiastic ovation.

Hogwood's spirited rendition of Prokofiev's ``Classical'' Symphony gave a strong finish to a program that opened with a routine performance of Haydn's Symphony No. 4 in D. In all, the program may have proved more interesting to listeners who share musicologist Hogwood's fascination with musical eras and their connections than to fans of Jarrett - or, for that matter, of Mozart, Prokofiev, or Haydn.

Next in the H&H season is the Jan. 29 debut of a three-concert series of chamber-music programs, being presented in various Boston churches.