Over the past 40 years, the Tokyo String Quartet has built a well-deserved reputation for rock-solid ensemble playing and immaculate, razor-edged precision. Those enviable qualities were often in evidence Wednesday night at the Kennedy Center's Terrace Theater, where the Tokyos opened the 31st season of the Fortas Chamber Music series.

But the group, to its credit, seems to be loosening up in middle age, allowing the individual players freer rein and focusing on spontaneity rather than polish. That agreeable approach, at any rate, was immediately clear in Mozart's "Quartet in G major, K. 387," which the Tokyos played with such relaxed affection that even its darker moments were filled with sunshine and puppies.

Roles were quickly defined: First violinist Martin Beaver provided the driving forward edge, anchored by Kikuei Ikeda's unflappable second violin; cellist Clive Greensmith was the soulful poet of the group; and violist Kazuhide Isomura - the only founding member left - played solidly, if a shade more woodenly than in his younger years.

Mozart's warmth and optimism were balanced by an absolutely gripping account of Karol Szymanowski's dark, brooding "Quartet No. 1 in C major, Op. 37." An early modernist work from 1917, it paints a lush, inward-looking and wildly imaginative landscape rife with eerie harmonics and muted sonorities - especially in the scherzo-ish last movement. It's a phenomenal piece, performed far too rarely, and the Tokyos played it ravishingly - helped partly by their extraordinary "Paganini" collection of Stradivarius instruments, which produced a sound so golden and seductive that it made your ears go all quivery with love.

The evening closed on a more upbeat note with a hugely entertaining romp through Antonin Dvorak's "Quartet in G major, Op. 106." Written in 1895, shortly after the composer returned to Prague from his years in America, it has fewer of the ersatz "Americanisms" of his earlier works and more of the earthy, thump-a-dump rhythms of Czech folk music, and (unlike the Szymanowski) there's nothing in it that might frighten the children. The Tokyo players had great fun with it, diving enthusiastically into its endlessly blossoming melodies and capturing the work's character with loose, unbuttoned elan.