Activities of serum glutamic pyruvic transaminase [SGPT], serum glutamic oxaloacetic transaminase [SGOT] and serum lactic dehydrogenase [LDH] were determined during and after surgery In a total of 18 patients with heart diseases Including 8 cases of congenital heart anomaly and 10 cases of acquired valvular diseases who undergone open heart surgery under cardiopulmonary bypass. The results obtained are as follows: 1]The average value of the activity of SGPT before the surgery was 27.94±12.24 units. The value was increased following the operation and reached to the maximum value of 43.83±37.02 units 2 days after the operation, which was slightly deviated from the normal range. The enzyme activity returned to the normal range of 32.82±23.61 units 4 days after the operation. 2]The average value of the activity of SGOT before the surgery was observed to be 30.5±11.29 units. The value increased during the operation and reached to the maximum value of 95.56±43.38 units. 2 days after the operation, the enzyme activity was gradually decreased thereafter and returned to the normal range 6 days after the operation. 3]The average value of the activity of serum LDH before the surgery was found to be 258.0±88.96 units. The value was increased during the operation and reached to the maximum value of 930.55± 258.23 units 2 days after the operation. The enzyme activity was gradually decreased thereafter and returned to normal range 7 days after the operation. 4]It was observed that the longer the time required for the extracorporeal circulation in the open heart surgery became, the greater the changed in activity of SGOT, SGPT and serum LDH were found.[KTCS 1982;1:46-52]