The blood levels of 6-keto-PGE1 alpha and thromboxane B2 were measured in the coronary sinus of 15 males during and just after a spontaneous myocardial ischemic episode. The comparison was made in 30 males with coronary heart disease in the presence of exercise-induced angina in whom coronary sinus blood samples were taken during myocardial ischemia provoked by pacing and 6 males suffering from cardialgias without signs of coronary atherosclerosis. The patients with spontaneous anginal attacks had lower baseline 6-keto-PGE1 alpha (179.0 +/- 47.8 pkg/ml) than those with exercise-induced angina (336.0 +/- 65.7 pkg/ml; p less than 0.1). This difference became greater during ischemia (165.0 +/- 49.0 and 350.0 +/- 69.5 pkg/ml, respectively, p less than 0.05) and just after its elimination (166.0 +/- 48.7 and 413.0 +/- 76.0 pkg/ml, respectively, p less than 0.05). Coronary sinus blood thromboxane B2 levels were not substantially different in the presence or absence of myocardial ischemia. Thus, a decrease in the prostacyclin-forming function of the coronary endothelium plays a definite role in the genesis of spontaneous myocardial ischemic episodes.