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Periodic pulses of intense ultrasound energy at 70 kHz with 5-ms duration at 3 MPa SPL applied to the exposed pig myocardium at physiologic rhythms were observed to produce cardiac pacing. Ten animals were successfully paced at pulse width and energy conditions above a certain threshold. Sigmoid strength-duration curves characterizing ultrasound's effectiveness in pacing were observed to resemble those of electrical stimulation. We suggest that bioelectrical stimulatory effects of ultrasound on the heart are a radiation pressure effect and a manifestation of the known myocyte sensitivity to stretch.