The likelihood of "professional musicians sitting down to play for the fun of it is uncommon," says violist Rachel Kam, who will join cellist David Premo and violinists Joel Berman and David Brewster in presenting "An Evening of Chamber Music" tonight at the National Institutes of Health.

The program, which consists of three string quartets; the Haydn String Quartet in C Major, Op. 54, No. 2; the Verdi String Quartet in E Minor; and the Mendelssohn String Quartet in E-Flat Major, Op. 12, is "performed rarely because of their difficulty," says Kam.

Playing -- and composing -- a string quartet is difficult, says Berman, because "you can't rely on instrumental colors, voices or picture thoughts as one does with symphony or opera . . . there are just musical ideas rather than pictorial ones." Tonight's particular performance is what Kam calls the epitome of musicianship. "We're sitting down together for the pure love of music . . . there's no competition between us as there sometimes is in professional groups; we're just trying to make the best music possible."

Promoted by the Foundation for Advanced Education in the Sciences at NIH, the performance is free and will begin at 7:30 p.m. at the Clinical Center's Masur Auditorium.