To test whether insulin is a regulatory factor of myocardial MB creatine kinase content, we investigated the correlation between the ability of insulin secretion and the MB fraction of cumulative CK released in patients with acute myocardial infarction. We analyzed 18 patients who underwent successful direct angioplasty within 10 hours of the onset of their first myocardial infarction. Exclusion criteria were age more than 75 years, heart failure, severe obesity, multivessel disease, and history of diabetes mellitus. Cumulative activity of serum MB CK divided by that of total CK was defined as MB%, which was considered to represent myocardial MB CK content. Two weeks or more after the onset of myocardial infarction, 75 gm oral glucose tolerance test with serial determination of plasma glucose and serum insulin (0, 0.5, 1, 2, 3 hours) was done. Urinary and plasma catecholamines and echocardiographic left ventricular (LV) mass were measured. MB% significantly correlated with insulinogenic index (r = 0.564, p = 0.019), insulin area (r = 0.594, p = 0.012), insulin area/glucose area (r = 0.630, p = 0.007), and urinary adrenaline (r = -0.542, p = 0.025) and tended to correlate with plasma adrenaline (r = -0.431, p = 0.084). Age, body mass index, infarct size, glucose metabolism, and LV mass were not significant univariate predictors of MB%. Multivariate analysis showed that the ability of insulin secretion contributed to MB% more than catecholamines did and that insulin area/glucose area was the strongest independent predictor of MB% (t = 3.01, p = 0.015). Thus MB fraction of cumulative CK released, indicative of Myocardial MB CK distribution, strongly related to the ability of insulin secretion in subjects without overt insulin resistance. Regulation by insulin of myocardial MB CK is suggested.