Until about 10 p.m. Saturday, a promising new chamber orchestra was giving its first performance in the Red Auditorium of the National Bureau of Standards in Gaithersburg. Then conductor Piotr Gajewski gave the downbeat for Beethoven's Second Symphony, and the promises began to be fulfilled.

Until then, the debut performance of the Montgomery Chamber Orchestra had not been bad but might have been better. There was a slight rigidity of beat in some of the dance movements of Telemann's Overture in D (actually a suite in the French style), but it was well played and the "Tourbillon" movement (a tiny tone poem depicting a tornado) had a touch of brilliance.

The coloratura singing of soprano Kathryne Jennings had sounded a bit forced and heavy, and the intonation was not always precise in the formidable opening aria of Bach's Cantata No. 51, "Jauchzet Gott." But that cantata is a hard way for a voice to begin an evening's work, and she improved notably and rapidly. She seemed quite at home in two arias from "The Marriage of Figaro" that brought the program to intermission, and Gajewski contoured the orchestra's dynamics sensitively to support her voice without covering it. Her voice was clearly audible and well balanced with the orchestra, but it never had the kind of impact it might have had in the Terrace Theater or the Wolf Trap Barns. It may be that this auditorium (like the Kennedy Center Concert Hall) is better for orchestras than for sopranos.

On the evidence of Saturday night's performance, most of the new 33-piece orchestra's rehearsal time must have been dedicated to Beethoven -- with results that clearly justified the effort. There were occasional lapses in ensemble playing, but they were barely noticeable in a distinctive, alert and highly idiomatic interpretation. There was a spring in the rhythms, a lilt in the melodies, a unanimity in the playing that often sounded as good as the work of long-established and internationally respected chamber orchestras -- the St. Paul, for example. The balance often gave more prominence to the (generally excellent) wind players than they would have in a symphony orchestra, but this is actually the way the music was intended to sound. Above all, conductor Gajewski has worked out a clear, highly detailed and cogent concept of the music; his statement had a lot of character, and the character was as much Beethoven's as his own.

The arrival of the Montgomery Chamber Orchestra is the latest episode in a trend that has become impressive in recent years: the development of a flourishing independent musical life in the Washington suburbs. The audience for its first concert was large (though the auditorium had seats for more), musically informed and enthusiastic. In the programs announced for the next three months, Gajewski is offering this audience a well-calculated blend of 18th-century and modern music. If he can maintain the standard set in his Beethoven Second, Montgomery County will have an orchestra it can support with pride.