Total CPK and the heart-specific isoenzyme of creatine phosphokinase(CPK-MB) values have been studied during operative and postoperative periods in 13 patients undergoing open heart surgery, using cellulose acetate electrophoresis. The results were: 1) Total CPK value of normal control group was 29±6.0 U/L (mean±S.D.) and only MM-fraction was observed. 2) Preoperative total CPK value was 49±50.7 U/L(mean±S.D.) and preoperative MB-fraction was not detected in most patients(l1/13), but detected in two patients(2/13). 3) Peak total CPK activity appeared from pump stop to 2nd postoperative day(39∼49 hours from pump stop) and gradually declined to normal activity by 6th postoperative day. 4) CPK-MB was first detectable during the period of cardiopulmonary bypass in about 46% of cases(6/13). 5) Peak MB isoenzyme was demonstrable during the period of pump stop to 1st postoperative day(16∼25 hours from pump stop) in 12 cases out of 13. 6) Duration of sustained CPK-MB showed no correlation with cardiopulmonary bypass and aortic cross-clamping time. 7) In cases showing relatively persistant CPK-MB, the peak total CPK and MB isoenzyme values were higher and remained for longer period of time, than transient CPK-MB group. 8) 2 cases showing preoperative CPK-MB died postoperatively.