Serum glutamic oxalacetic transaminase concentrations rise sharply after myocardial infarction produced by bead embolization of the coronary tree in the closed chest dog. The peak concentration is reached in 9 to 23 hours post injury. Rises 20 to 30 times greater than normal may be obtained and are directly correlated with the amount of infarction estimated at autopsy. Serial serum transaminase levels may provide a clinical tool to supplement the electrocardiogram in the diagnosis of acute myocardial infarction as well as provide a roughly quantitative estimate of the amount of myocardium involved.