When the Viennese musical tradition -- probably history's most productive one -- started trying to find its way after the turn of the 20th century, it found itself a colossus that didn't know where to go. What was left to do with the forms that had taken shape in the music of, say, Haydn and Mozart and not really changed all that much through Mahler? Where did one extend that radical step into chromatic harmony that Wagner took in "Tristan"? Was there anything left to do with the extraordinary tonal resources of the modern orchestra after Strauss' tone poems? Was this the end?

The answer was both no and yes. No, the Viennese tradition did not continue on the Mozart-to-Mahler course. And, yes, there was something left to say.

That was what made Saturday night's concert at the Kennedy Center by the Theater Chamber Players so fascinating. It was not just the scholarly excavation of musical fossils that was behind this evening of virtually unknown pieces by the less than well known Alexander von Zemlinsky (1871-1942) and Erich Wolfgang Korngold (1897-1957).

These composers were two talented men who chose to wrestle with the post-Romantic dilemma in a way less radical than, say, the atonality of Arnold Scho nberg -- who was, it happens, Zemlinsky's brother-in-law.

The Chamber Players' loving and painstaking exploration of some of these undervalued works (indeed, they proved to be quite substantial) made this one of the most absorbing concerts of the season -- especially in these performances, with Leon Fleisher's piano playing in the concluding Korngold Suite simply breathtaking.

Take a simple example of what these works are like, and of their variety. Zemlinsky was a prolific songwriter, and the program opened with seven of them uncovered in their manuscripts at the Library of Congress. Each was lovely, as performed by soprano Jeannette Walters and pianist Dina Koston. But the last three were especially striking -- and particularly significant -- in the way they showed how modern cosmopolitanism would both enrich and bring to an end the Viennese tradition.

The first two came from, believe it or not, settings of German translations of Langston Hughes (Zemlinsky did not know the Harlem Renaissance in English). Obviously, he doesn't catch the verbal idiom of "Elend" ("Misery"), which Hughes conceived as the lament of a woman whose man "done her wrong." But the composer memorably mixes touches of the blues with the great tradition of the lied, and all the more so for its briefness and conciseness. The following song, "Afrikanischer Tanz," was somewhat similar. It reminded one of Kurt Weill, but had a little more expressive discipline.

Then, in the last song, "Wanderer's Nachtlied," Zemlinsky returned to the tradition whence he came, text by Goethe and the sound of Hugo Wolf. Wolf would hardly have been ashamed of this one.

The songs give one the idea of what both composers were doing -- writing in the language of our century, but without a radical break from the past.

Zemlinsky's Third String Quartet did the same, but on a much larger scale. Its sounds were leaner than one might have heard in the previous century (lots of pizzicati to dilute certain textures, as well as some playing above the bridge). Moods were more ambivalent. But the lyricism was unmistakable, including one especially gorgeous rapt moment suspended in melodic time by the first violin.

Korngold's lengthy and bold Suite for Piano (left hand), Two Violins and Cello concluded the program (a product of 1930, before he went to Hollywood and made a fortune writing movie music). In five movements, it is sometimes as if the compositional range of Mahler were combined with the sound of Schubert. There are, among other things, an understated little waltz, a truly lovely little lied and a Mahlerian "Groteske." But perhaps the most striking moment is an opening recitative-like section for solo piano, played with extraordinary force and discipline by Fleisher.

The other name on the program was Brahms, in two of his dark late songs for soprano, viola and piano. They were magnificent. But the compositions by the two composers whose fate it was to follow him did not suffer all that much by comparison.