The effects of prostaglandin E1 on pulmonary circulation and left ventricular performance have been studied in 20 patients with mitral valve disease and pulmonary hypertension. Prostaglandin E1 was administered intravenously over a period of 30 minutes. The dose used was 0.01 microgram/kg per min during the first 15 minutes and 0.02 microgram/kg per min subsequently. The first dose led only to an insignificant fall in left ventricular end-diastolic pressure. Infusion of prostaglandin E1 in a dose of 0.02 microgram/kg per min resulted in a significant fall in the pulmonary arterial pressure (P less than 0.001), total pulmonary resistance (P less than 0.001), left ventricular end-diastolic pressure (P less than 0.001), and aortic pressure (P less than 0.01), and an increase in the pulmonary blood volume (P less than 0.01), cardiac index (P less than 0.01), and heart rate (P less than 0.05). No significant differences were noted in stroke volume index or left ventricular dP/dt at 50 mmHg after prostaglandin E1. These results indicate that exogenously administered prostaglandin E1 causes active vasodilatation of the pulmonary vascular bed and has no inotropic action on the cardiac muscle.