The effects of vibrations on myocardial contractility have been tested in isolated rabbit papillary muscles. Sinusoidal longitudinal oscillations were found to inhibit active force in the paced preparation to an extent which depended on vibration amplitude and frequency. The inhibitory effect of vibrations on myocardial contraction resembled that previously seen in other types of muscle. Vibration during the inactive phase did not alter passive muscle tension. When the vibrator was controlled by phonocardiographic recordings from a normal subject or from patients with congenital aortic stenosis, pronounced inhibition was obtained only by vibrations corresponding to a systolic murmur. It is concluded that the myocardium is sensitive to oscillating length changes. If the ventricular muscle in vivo is exposed to vibrations during systole the cardiac function might be seriously interfered with.