Twenty-three patients exhibiting coronary insufficiency on exercise have been studied at rest and at exercise prior to and following sublingual administration of nitroglycerin. The most consistent and marked effect of nitroglycerin was the reduction in pulmonary artery pressure, total pulmonary resistance, and right ventricular work at rest and during exercise. Our patients were grouped according to the degree of left ventricular failure as indicated by the abnormality of rise in pulmonary artery pressure during exercise. It became evident that nitroglycerin improved left ventricular function in terms of increased output and lessened pulmonary artery pressure in cases in which failure was most severe.