You really have to hear Rachmaninoff's "Vespers," all hour and 10 minutes of its complex Russian notes for huge a cappella chorus, to realize what an extraordinary departure from musical idiom this work is. Mstislav Rostropovich gave one of those rare opportunities in a remarkable performance last night by an enlarged Choral Arts Society at St. Matthew's Cathedral.

Rest assurred, this austere and very complex setting of the liturgy from all-night vigils that took place on the eves of Russian Christian festivals is no mere curiosity. It came from his maturity (1915), and that it sounds more like Stravinsky than Rachmaninoff tells us how little we still know of some of the reaches of Russian music that Rostropovich is gradually reviewing here. In a way, the relative neglect of "Vespers" and other such works is yet another tragedy of the Revolution. Rachmaninoff had to flee West, and this fascinating side of his character did not develop further.

Much of "Vespers' " often gripping drama is in the stark contrasts of vocal ranges and timbres -- and between quiet and gigantic sonorities. This was the performance for which Choral Arts Society head Norman Scribner advertised for some lower bass voices of the Russian order (the bottom note is a flabbergasting B-flat beneath low C). Some of the 15 sections are tranquil and meditative, such as the gorgeous little "Ave Maria." But then there is also a brilliant "Gloria," the second of two, that is on the scale of the great settings of the Latin Mass. Its ending, especially, is magnificent.

There are two soloists, in this case contralto Maureen Forrester and tenor John Alexander. They sang very well, but Rachmaninoff really didn't give them all that much to do.

This was the second time Rostropovich has done "Vespers" here, this time a benefit for the Choral Arts Society. Rostropovich, true believer that he is, puts all the intensity of his deep feelings into the work. And by now he seems to have made true believers as well of the 200-plus singers.