Activity from left ventricular receptors with non-medullated afferents was recorded in the right cervical vagus in anesthetized, thoracotomized cats. Probing of the open heart demonstrated that the receptors were distributed throughout the free wall and the interventricular septum. Fibers from posterior receptors pass along the posterior descending coronary artery and the lateral surface of the right atrium to join the right main cardiac nerve; those from the anteriolateral region pass behind the aorta and the pulmonary trunk. The control receptor discharge was 1.4 (range, 0-6) impulses/sec at a mean left ventricular end-diastolic pressure (LVEDP) of 4.5 (range, 2-10) mm Hg. It was observed that receptor discharge increased with progressive increase in LVEDP produced either by transfusion or by aortic occlusion. At a mean LVEDP of 8 mm Hg, the mean discharge rate was 3.2 impulses/sec, and at 16 mm Hg it was 7.0 impulses/sec. It was also found that propranolol reduced and isoproterenol increased the discharge frequency at any given LVEDP. Measurement of total conduction times indicated that during the increase in LVEDP the receptors were activated principally in the systolic portion of the cardiac cycle. Despite this there was no obvious relationship between the discharge frequency and left ventricular systolic pressure.