The specific binding sites of125I-insulin in platelets from nondiabetic patients with acute myocardial infarction and unstable angina pectoris was significantly decreased (192 ± 84/cell) during the acute ischemic condition when compared with normal platelets (496 ± 76/ cell; p < 0.001, n = 9). A relatively low mean plasma insulin level (20 μU/mL) with an elevated mean glucose level (130 mg/dL) was noted. The decreased binding of insulin and the relatively low mean plasma insulin level during the acute ischemic condition improved into the normal range during 8 to 12 weeks of recuperation. The decrease of insulin binding sites in the acute phase and their subsequent increase during recovery were directly related to the hormone-induced increase in sensitivity to prostaglandin Ex through the stimulation of adenosine 3′,5′-cyclic monophosphate formation in platelets by the prostanoid.