The changes of serum creatine phosphokinase [CPK], serum lactic dehydrogenase [LDH], serum glutamic oxaloacetic transaminase [GOT] and serum glutamic pyruvic transaminase [GPT] were studied in a total of 82 cases who underwent open heart operation under the cardiopulmonary bypass at the Department of Thoracic and Cardiovascular Surgery, Chungnam National University Hospital. The results were as follows 1. The average values of CPK and COT after the operation were reached to the maximal values on the first postoperative day, which were returned to the normal range until the fifth postoperative day. The average values of LDH and CPT after the operation were reached to the maximal values on the first and second postoperative day respectively, which were significantly increased until the seventh postoperative day. 2. In the relationship of the serum enzymes and duration of the extracorporeal circulation, the values on the group over 90 minute of the extracorporeal circulation were more increased than on the group below 90 minute of the extracorporeal circulation. 3. In the relationship of the changes of the serum enzymes and congenital heart diseases and acquired heart diseases, there were no significantly differences in the values between the two groups.