In his finest music, Beethoven had no pity on his performers, and in both the Ninth Symphony and the "Missa Solemnis," he asks singers to explore vocal extremes that are both intellectually and physically exausting.

It was to the enormous credit of the Oratorio Society of Washington, in their performance of the Mass at the Kennedy Center Concert Hall last night, that they sounded as fresh at the last Dona Nobis Pacem as they did in the opening Kyrie.

Conductor Robert Shafer prepared a well-groomed performance of this complex piece, a performance that featured an excellent solo quartet, a well-rehearsed, if somewhat understaffed orchestra, and a chorus that managed the extended fugal choruses with unusual clarity and the declamatory sections with conviction. The tenor and bass sections were particularly impressive in their exposed solo lines, and the beginning of the "Et Incarnatus Est," scored for tenor solo but sung, in this performance, by the whole section, was gorgeous.

On the whole, Shafer opted for a thoughtful, rather than dramatic, interpretation, and only in the "Et Vitam Venturi," where the tempo was uncomfortably slow and the vocal line unaccountably detatched, did he reach for effect and not achieve it.

The major soloistic duties fell to contralto Gretchen Greenfield and tenor Gene Tucker, both of whom were outstanding. Greenfield has a huge, rich voice that manages to be both flexible and accurate as well, and Tucker, always musical, has, over the years, developed a much bigger sound. Soprano Carmen Pelton and bass Stephen Saxon fulfilled their smaller assignments admirably.

The performance will be repeated tomorrow.