Intraarterial or intravenous infusion of prostacyclin at doses 2, 5, and 10 ng/kg/min into 5 subjects without evidence of coronary heart disease or cardiac failure led to marked fall in peripheral, total pulmonary, and capillary wedge resistances. This was accompanied by a moderate drop in intraarterial blood pressure and the acceleration of heart rate leading to compensatory increase in cardiac output. Both stroke volume and mean right atrial pressure remained unchanged. No side effects occurred. Prostacyclin appears to act predominantly on resistance vessels. The hemodynamic effects produced by prostacyclin in man might be of clinical interest in treatment of conditions associated with significant rise in vascular resistance, heart failure, and in limitation of infarct zone during the early stage of acute myocardial necrosis.