Elevation of serum levels of enzymes due to myocardial necrosis is considered to be a specific finding for myocardial infarction. This study was designed to determine the changes in the levels of LDH, SGOT enzymes, LDH1, CK-MB isoenzymes and myoglobin in patients who have had coronary bypass grafting and who have had atriotomy for surgical correction without any evidence of myocardial infarction diagnosed by clinical, hemodynamic and electrocardiographic findings. It was found that CK-MB, LDH, LDH1 and myoglobin showed significant elevations which mimicked a perioperative myocardial infarction. SGOT also increased, but remained within the normal range. CK-MB, LDH and LDH1 increased more in the coronary bypass grafted patients whereas myoglobin increased in the patients who had atriotomies. It was concluded that in order to demonstrate a perioperative myocardial infarction it is necessary to determine at least two of these enzymes for a long period and to correlate these results with the clinical, hemodynamic and electrocardiographic findings.