Intra-arterial or intravenous infusion of prostacyclin at three dose levels (2, 5, and 10 ng/kg per min) in 10 subjects without evidence of coronary heart disease or cardiac failure, led to a distinct fall in peripheral and total pulmonary vascular resistances. This was accompanied by a drop in intra-arterial blood pressure, and the acceleration of heart rate. Stroke volume, cardiac output, mean right atrial pressure, and left ventricular end-diastolic pressure showed no significant changes. Except for sporadic headache no side effects occurred. Prostacyclin appears to act predominantly on resistance vessels. The haemodynamic effects produced by prostacyclin in man might be of clinical interest in the treatment of conditions associated with a significant rise in vascular resistance.