Cardioplegia and myocardial protection were performed under cardiopulmonary bypass during open heart surgery with the use of St. Thomas Hospital cardioplegic solution [4 [C] for the coronary artery perfusion and normal saline solution [4[ c] for the topical cardiac cooling. To maintain the state of myocardial protection, coronary artery reperfusion was carried out using St. Thomas Hospital cardioplegic solution at the interval of 30 minutes. A total number of patients studied were 57 cases, including 37 cases of correction for congenital anomalies and 20 cases for acquired heart diseases. Cardiopulmonary bypass time during the surgery was observed to be average of 87.89*47.55 hours, aortic cross-clamping time [ACCT] to be average of 76.68*44.27 hours raging from 30 to 191 minutes. In order to evaluate the effects of myocardial protection in the surgery, serum enzyme levels were determined. To observe the relationship between ACCT and myocardial protection effects, patients studied were divided into the following 3 groups. I group: ACCT 60 minutes, II group: ACCT 90 minutes, III group: ACCT over 91 minutes [1] SGOT; The positive value [increased over 200 units] for ischemic myocardial injury during operation was observed in 11 cases [19.3% of the total] of the total patients studied, of which 4 cases [13.3%] in I group, 1 case [10.0%] in II group, and 6 cases [35.3%] in III group. [2] LDH; The positive value [increased over 900 units] for ischemic myocardial injury during operation was observed in 9 cases [15.7% of the total] of the total patients studied, of which 2 cases [6.6%] in I group, 1 case [10.0%] in II group and 6 cases [35.3%] in III group. [3] CPK; The positive value [increased over 800 units] for ischemic myocardial injury during operation was observed in 10 cases [17.5% of the total] of the total patients studied, including 4 cases [13.3%] in I group, 1 case [10.0%] in II group, and 5 cases [29.4%] in III group [4] The myocardial protection method used in the present study was demonstrated to be effective for the myocardial protection in the surgery with ACCT of up to 90 minutes. A few ischemic myocardial injury were observed in the surgery with ACCT over 91 minutes, but no significant cardiac dysfunction was noted. The surgery with ACCT of up to 191 minutes did not appear to give rise any significant interference with postoperative recovery.