Simultaneous electrocardiograms, phonocardiograms, and echocardiograms were recorded in 21 patients with mitral valve prolapse. Four patients with holosystolic mitral valve prolapse on echocardiogram had smaller resting end-diastolic volumes than the remaining 17 patients with late systolic echocardiopraphy prolapse (p greater than 0.01). Thirteen of the 17 patients with late systolic prolapse had phonocardiograhically recorded auscultatory phenomena. The initial vibrations of the auscultatory phenomena occurred after the onset of echocardiographic prolapse, but prior to maximal echocardiographic mitral valve prolapse. Amyl nitrite was administered to all patients. Three of the 17 patients with late systolic prolapse developed holosystolic prolapse, while the remaining 14 retained the late systolic prolapse pattern during amyl nitrite inhalation. In these 14 patients, the onset of mitral prolapse occurred earlier in systole due to decrease in the duration of systole prior to onset of mitral valve prolapse (p greater than 0.001). This corresponded with the occurrence of auscultatory phenomena earlier in systole. Twelve patients had left ventricular volumes recorded during amyl nitrite inhalation and all showed a decrease in left ventricular volumes (greater than) 0.001). These findings confirm the temporal relationship of mitral valve prolapse and onset of auscultatory phenomena in these patients. It suggests that the movement of auscultatory phenomena earlier in systole during amyl nitrite inhalation is related to earlier prolapse of the mitral valve, and that a decrease in ventricular valume is a tenable explanation for the earlier onset of prolapse.