In open-chest dogs, the peripheral circulation was carried on a heart-lung machine. The pulmonary artery was obstructed and the left atrium and the right ventricle were drained into the venous reservoir of the machine. A balloon in the bloodless left ventricle permitted its distention. Pressures were recorded in the left ventricle and the aortic arch or a femoral artery. After distention of the left ventricle, the left ventricular diastolic pressure rose, the systemic arterial pressure fell, and bradycardia occurred. Distention of the left ventricle also caused reflex dilation of systemic veins. These effects were reversible and were abolished by section of the vagi. They are attributed to receptors in the myocardium of the left ventricle. It is considered likely that these reflex effects of left ventricular distention contribute to the mechanism of cardiogenic shock.