Prostacyclin is a powerful inhibitor of platelet aggregation and induces relaxation of vascular smooth muscle. We have studied the coronary effects of high local concentrations of prostacyclin in 17 patients during the early stages of acute myocardial infarction. We infused graded concentrations of intracoronary prostacyclin and compared the effects to those of intracoronary isosorbide dinitrate before and after an intracoronary infusion of streptokinase. Considerable dilation of the infarct-related stenosis may follow local infusions of prostacyclin in some patients. In general, however, dilation of the infarct-related stenosis or coronary recanalization did not result from infusion of prostacyclin.