In order to assess the correlation of the myocardial damage and the duration of cardiopulmonary bypass, measurement of creatine kinase [CK], lactate dehydrogenase [LDH], asparatate aminotransferase [AST], and MB band of CK [CK-MB] were carried out on the first, third, fifth, seventh, and ninth day in 44 patients following open heart surgery [POD 1,3,5,7,9]. And the patients were divided into three groups according to the duration of aortic cross clamp time [ACT]: Group I [ACT< 60 minutes. n=19], Group II [60 minutes < ACT< 90 minutes, n=7] and Group III[90 minutes > ACT, n=18]. 1. The leakage of CK in total patients increased to the highest level at POD 1, with rapid decrease and recovery at POD 7. The leakage of CK in Group III were greater than in Group I from POD 1 to POD 3 [P < 0.01]. The recovery time of CK level was shorter in Group I [POD 3] than in Group II and III [POD 7]. 2. The serum levels of LDH in total patients increased to the highest level at POD 1, with slow recovery until POD 9. The levels of LDH in Group III were higher than in Group I until POD 9 [P < 0.005]. The levels of LDH in Group I and II recovered but not in Group III. 3. The serum levels of AST in total patients increased to the highest level at POD 1, with rapid decrease and recovery at POD 7. The levels of AST in Group III were greater than in Group I from POD 1 to POD 5 [P < 0.05]. The recovery time of AST level was shorter in Group I and II [POD 5] than in Group III [POD 7]. 4. The positive cases for CK-MB in 36 patients were 22 [61.1 %] as a whole, 5[41.6%] in Group I, 4[57.1 %] in Group II, 13[76.4 %] in Group III at POD 1, and a case in each group at POD 3, and only a case in Group Ill at POD 5. It is concluded that the myocardial injury was closely related with the duration of cardiopulmonary bypass in open heart surgery.