Studies were made on 21 patients, 8 without any symptoms of left ventricular failure, group 1, and 13 with clinical symptoms of heart failure, group 2. Cardiac output, mean aortic and left ventricular pressures (using catheter tip micromanometer), and ventricular volume (obtained from left ventricular cineangiograms) were measured before and during nitroprusside infusion. The heart rate did not change in either of the groups. Only in group 2 did cardiac index and stroke volume increase significantly. Mean aortic pressure and total systemic vascular resistance decreased significantly in both the groups. Left ventricular end-diastolic pressure decreased significantly in both the groups, but this decrease was greater in group 2 (9 mmHg compared with 3 mmHg for group 1). The decrease in the left ventricular end-diastolic volume was similar in both the groups. The decrease in left ventricular end-systolic pressure was greater in group 1, but the decrease in the left ventricular end-systolic volume was greater in group 2. These facts are explained by the differences in the active and passive left ventricular pressure-volume relations in the two groups.