Written when he was 16, Mendelssohn's Octet is one of the glories of chamber music, and stupefying proof that age is no barrier to genius. The work's technical intricacies tax even the finest string players, who (to quote the musicologist Donald Francis Tovey) "might easily practice it for a lifetime without coming to an end of their delight in producing its marvels of tone color."

Saturday night at the Kreeger Museum, two string quartets--the Ying and the St. Lawrence--joined forces in an exhilarating performance that stretched this piece to its limits and captured intact Mendelssohn's wicked, boyish glee. The ensemble's pinpoint accuracy and immaculate balance bespoke not only fine musicianship but a rare kinship of spirit.

Borrowing St. Lawrence violist Lesley Robertson, the Ying opened the program with Mozart's G Minor Quintet, K. 516, which arguably stands alone as his most expressively passionate chamber composition. Its aching counterpoint, irregular rhythms and poignant melody reach forward to Schubert, and the Ying Quartet's richly arched phrasing seemed just right for it.

The St. Lawrence Quartet, plus the viola of Phillip Ying and the cello of David Ying, gave Brahms's early String Sextet, Op. 18, a high-voltage reading without subverting its songful inclinations.