Voluntary patients with a history of myocardial infarction and with typical effort angina underwent catheterization of the coronary sinus and a brachial artery. Healthy young males, serving as controls, were subjected to the same procedure. Arterial and coronary venous blood was drawn at rest and during atrial pacing to angina (patients) or to a heart rate of 140 beats/min (healthy volunteers) for analysis of 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) and prostacyclin-like activity (PILA). 6-Keto-PGF1 alpha levels were measured using radioimmunoassay; PILA in the blood was assayed by rapid preparation of platelet-rich plasma followed by determination of the ADP-induced platelet aggregation. Increased arterial levels of PILA and of radioimmunoactive 6-keto-PGF1 alpha (RIA-6-keto-PGF1 alpha) were observed in the patients at rest as well as during pacing. No obvious release of RIA-6-keto-PGF1 alpha occurred at rest, either in the patients or in the controls. However, during pacing, increased amounts of RIA-6-keto-PGF1 alpha appeared in the coronary venous blood of the patients. The results demonstrate that an increased cardiac prostacyclin formation prevails in patients with signs of impaired coronary flow and suggest that ischemic heart disease is characterized by an insufficient vascular response to this vasodilator prostaglandin rather than by its insufficient endogenous production.