THE diagnosis of acute myocardial infarction when it occurs intra-operatively or during the early postoperative period is often difficult to establish. The pain patterns, physical signs, hemodynamic changes, electrocardiographic aberrations, and serum enzyme responses characteristic of an acute myocardial infarction may be modified or completely masked by the early postoperative state. The patient's response to the anesthetic agent, operative trauma, administered drugs, and imbalance between blood loss and replacement may in certain instances obscure the diagnosis of an acute myocardial infarction which occurs intraoperatively or during the early postoperative period. In the present study the responses of the serum levels of glutamic oxaloacetic transaminase (SGOT), total lactic dehydrogenase (LDH), and the heat stable fraction of lactic dehydrogenase (HS-LDH) following certain types of operative procedures were measured and were compared to normal serum values and to values observed following an acute myocardial infarction in patients who were not operated upon. It