The frequency spectrum of the first heart sound (S1) was measured noninvasively in 54 patients with porcine bioprosthetic valves inserted in the mitral position. Phonocardiograms were recorded on magnetic tape on line with a signal processor with which the frequency spectrum and peak frequency of S1 were determined. In 19 patients with normal natural mitral valves, the apparent peak frequency within the range of measured frequencies of S1 was 46 +/- 2 Hz (mean +/- SEM). In 11 patients with porcine bioprosthetic valves implanted in the mitral position for less than or equal to 11/2 years, the apparent peak frequency of S1 was 43 +/- 3 Hz, which was not significantly different from S1 in patients with normal mitral valves. However, in 33 patients with porcine bioprosthetic valves in place 5-7 years, the apparent peak frequency of S1 was higher, 67 +/- 2 Hz (p < 0.001). In patients in whom the porcine bioprosthetic valve was implanted 5 years or longer, the frequency spectrum of S1 showed a greater proportion of sound energy at frequencies between 50-200 Hz compared with patients in whom the prosthetic valve was implanted 11/2 years or less. changes of the frequency of S1 in these patients may be a manifestation of stiffening of the valve as a result of early degenerative changes.