Data are presented on the base-line level of general insulin activity and free serum insulin determined by biological techniques in 45 patients during the acute and subacute phases of their myocardial infarction. Patients with normal body weight demonstrated a reduction of the insulin activity parameters during the initial days of the disease, the reduction being most distinct in a complicated course of the acute period. The indices of free insulin varied widely, and the mean values were close to the controls, although the majority of patients had a reduced level of free insulin in blood. During the subacute phase of myocardial infarction the indices of insulin activity and free insulin did not differ from the control figures in the majority of patients. The general insulin activity remained reduced during the subacute period as well if the course of the disease was complicated by the development of circulatory insufficiency. In obese patients the indices of insulin activity and free insulin exceeded during the acute period of myocardial infarction those of the control, and during the subacute stage of the disease they became even higher.