Fifty one consecutive patients undergoing open heart surgery, twenty eight congenital and twenty three acquired heart disease, were studied between May and August 1979 in Dept. of Thoracic and Cardiovascular Surgery SNUH. During the same time 10 patients of PDA were included in this study as control group. Four out of fifty one OHS patients, two ASD and two pulmonic stenosis patients, were operated without aortic cross-clamp. In all patients, serial determination of total level of creatine phosphokinase [CPK], lactic dehydrogenase [LDH], glutamic oxaloacetic transaminase [SGOT] were made preoperatively, operative day [immediate post-op], and post-operative days up to 7th day. Electrocardiograms were also evaluated serially. Open heart surgery patients were divided into two groups; Group A was aorta clamp time beyond SO minutes, and Group B was below 50 minutes. The peak level of each enzyme was compared, and electrocardiographic changes were also compared between groups. Although the electrocardiographic changes were more frequent in Group A [50%] than Group B [24%], the peak levels of each enzymes were almost same in Group A and Group B.