After two weeks of fluffy contemporary repertoire led by yuppie guru Christopher Keene, the CSO gets back to serious business this week with a program of meaty contemporary works led by the always interesting music director of the Saint Louis Symphony Orchestra, Leonard Slatkin. Of particular interest is the most important orchestral work of the young Bartok, the rarely performed symphonic poem Kossuth, Bartok's enthusiastic response to his first hearing of Richard Strauss's Also sprach Zarathustra. Here, Bartok's music bursts forth in a blaze of heroic triumph, evoking the 1848 Hungarian Revolution and its leading figure, Lajos Kossuth. The later Bartok is represented by a series of delightful Romanian folk dances arranged by the composer. Also on the program is the rhythmic Janacek Sinfonietta, the bouncy finale of which served as the basis for Emerson, Lake and Palmer's "Knife-Edge," and the Chicago premiere of a new work by American composer Stanley Wolfe, a violin concerto. The concerto was written for Mark Peskanov, who will be the soloist here, and who successfully premiered it in New York with Slatkin last month. Wolfe and Slatkin will lead a discussion of the piece before the Tuesday performance, at 6:30 in the ballroom of Orchestra Hall. Today, 1:30 PM, Saturday, 8 PM, and Tuesday, 7:30 PM, Orchestra Hall, 220 S. Michigan; 435-6666.