The changes in myosin light chain I (MLC I) following aortocoronary bypass were studied in 31 patients and the curves of these changes were classified into three different patterns. A peak level of 14.7 +/- 1.54 ng/ml was seen in seven patients on postoperative day (POD) 2 which decreased suddenly to less than twice the normal value by POD 7 (group 1). Another 19 patients showed a peak level of 20.7 +/- 16.5 ng/ml on POD 5, which decreased slowly and was still high even by POD 7 (group 2). The remaining five patients developed only a slight increase in the MLC I level after the operation, with a peak value of 5.5 +/- 0.8 ng/ml (group 3). Creatine kinase myocardial band and glutamic oxaloacetic transaminase also remained low in this group. No correlation existed between the peak value of MLC I and the aortic cross-clamping time, or between the peak value of MLC I and the cardiopulmonary bypass time. Furthermore, the difference in cardiac output before and after the operation did not significantly differ among the three groups. These findings indicate that the measurement of MLC I is useful for diagnosing perioperative myocardial damage and may also be useful in the study of myocardial protection.