It's not easy keeping up with Robert Shaw these days, at least on records, where, since stepping down as music director of the Atlanta Symphony, he continues to enjoy perhaps the most prolific Indian summer of any conductor since the late Bruno Walter.

Just the same, two of the above three releases seem to me to merit special attention. First the Walton/Bernstein disc, not so much for "Belshazzar's Feast" or "Chichester Psalms," each of which has been engraved to better effect elsewhere, but for the first-ever recording of Bernstein's "Missa Brevis."A reworking of the composer's music for the Jean Anouilh/Lillian Hellman play "The Lark" (based on the trial of Joan of Arc), it was made at Shaw's suggestion and premiered by him in 1988, on the eve of his retirement. The result is a somber evocation of antiquity that nonetheless embodies a strong rhythmic sense - e.g., the impetus of the "Benedictus" and dancelike air of the "Dona Nobis Pacem," complete with tambours.

It also receives the finest performance of the three, the otherwise appealing "Chichester Psalms" missing the jazzy swing of Bernstein's own recording, on CBS, or even Richard Hickox's, on MCA. Nor, for all its precision and spectacular low end, does this "Belshazzar" quite match the idiomatic feel some others have brought to it, principally Andre Previn and the composer himself. Moreover, why does Shaw feel impelled to bring the chorus back on the final chord, something not authorized in any score I have seen?

The same comparatively straightforward approach can be heard in the Poulenc collection. Which is to say even John Rutter finds more wit and animation in the Four Christmas Motets, as do a number of other conductors in the Mass. But if Shaw's essentially non-linear style is to your taste, there is little to fault in the singing, remarkable for its shading and clarity. Especially in the Saint Francis Prayers, where the men's voices respond gorgeously to the cathedral-like atmosphere.

Where that ambiance really pays off, however, is in the Rachmaninoff Vespers, likewise recorded last year in a French church. Indeed, this has to rank among the conductor's very finest recordings, displaying not only his customary purity and precision but a feeling for these nocturnal chants I find every bit as idiomatic as Mstislav Rostropovich's, on his Erato recording of some years back.

I don't know that either quite supersedes the old Sveshnikov recording, from Melodiya, which boasts in addition to the authentic Eastern Orthodox sound those bottomless basses only the Slavs seem capable of producing in any quantity (cf. the low B flat at the end of the "Nunc Dimitis"). Nor am I sure why Shaw dispenses with the solo alto in No. 2, when he includes the tenor in Nos. 4, 5 and 9.

But Shaw's basses are no slouches either, and even the Russians do not surpass the massed blend his singers achieve at either end of the dynamic scale. In addition to which his generally slowish tempos, more appropriate here than in the Poulenc pieces, let everything register, from the reverential hush of the quieter sections to those great waves of sound rising majestically to the heavens.

Not to be missed.