During routine cardiac catheterization often patients with ventricular septal defect, the hemodynamic parameters were measured in the resting state and during sodium nitroprusside infusion at an average rate of 8/zg/kg/min. Two distinct hemodynamic groups could be identified. Group I characterised by elevated left ventricular filling pressure showed a consistently favourable response to sodium nitroprusside infusion with a decrease in the left ventricular tilling pressure, the pulmonary arterial pressures and the left to right shunt. The favourable effect was most pronounced in patients who had pulmonary arterial hypertension in addition to elevated left ventricular filling pressures. Group II patients characterised by normal left ventricular filling pressures showed a variable response to sodium nitroprusside administration. The left ventricular end-diastolic pressure fell or was unaltered. The pulmonary arterial pressures and pulmonary vascular resistance fell in those with pulmonary arterial hypertension. Patients without pulmonary arterial hypertension showed an increase in the magnitude of left to right shunt. On the basis of our data, we feel that vasodilator drugs have a specific role in the management of congestive heart failure secondary to large left to right shunts. The present study strongly indicates the need for further investigations utilizing orally effective vasodilators in patients with congestive heart failure due to left to right shunts.