This study was performed to observe the effect of myocardial ischemia on the patients undergoing cardiopulmonary bypass (CPB) operation. For this aim, superoxide dismutase (SOD) activities, reduced glutathione (GSH) and lipid peroxide (LP) levels were determined in blood samples which were obtained from the coronary sinus. Sampling times were as follows: (1) Before CPB. (2) Immediately after CPB. (3) Fifteen minutes after the second specimen. (4) Thirty minutes after the second specimen. SOD activities of these groups were 5135.10 +/- 278.51 U/g Hb, 3505.64 +/- 302.09 U/g Hb, 4206.55 +/- 272.25 U/g Hb, 4707.20 +/- 270.91 U/g Hb respectively. Also the LP levels were 1.90 +/- 0.29 nmol MDA/ml, 4.37 +/- 0.52 nmol MDA/ml, 4.09 +/- 0.39 nmol MDA/ml, 2.74 +/- 0.30 nmol MDA/ml respectively. GSH levels were slightly increased during ischemia and reperfusion; 103.27 +/- 5.18 mg/dl, 125.00 +/- 10.36 mg/dl, 125.00 +/- 6.61 mg/dl, 111.18 +/- 8.22 mg/dl respectively. SOD activities were reduced significantly in second group (p < 0.001), in third and fourth groups (p < 0.01) as compared with control group. Also LP levels were increased significantly in all groups (p < 0.001) as compared with controls. Our results confirm the generation of oxygen free radicals from ischemia and reperfusion of heart during CPB. Also it appears that oxygen free radical generation exceeds the capacity of intracellular SOD, which is the most important scavenger of free radical in the cell.