The potent antioxidative potential of propofol during cardiopulmonary bypass (CPB) in adults was investigated. The selected 30 patients receiving open heart surgery under CPB were randomly divided into group A and group B. The patients in the group A and group B were given propofol (0.1 mg.kg -1 .min -1 ) and fentanyl (5 μg.kg -1 .min -1 ) respectively to maintain anesthesia after aorta was cross clamped. Blood samples were drawn pre anesthesia, pre CPB, at 30 min of CPB, at the end of CPB, at 1 h after CPB, at the end of operation, at 12 and 24 h postoperatively. RBC suspension was prepared and erythrocyte glucose 6 phosphate dehydrogenase (G 6 PD) and phosphofructokinase (PFK) activities, total erythrocyte reduced glutathione (GSH) and oxidized GSH (GSSG) were assayed and GSH/GSSG ratio was calculated. In the group A, G 6 PD and PFK activities and GSH/GSSG ratio were almost uneventfully during CPB and postoperatively. In the group B, G 6 PD activity was increased and PFK activity and GSH/GSSG ratio decreased significantly from 30 min of CPB until 12 h postoperatively. It was demonstrated that propofol could obviously attenuate free radical activity during CPB, while fentanyl has no effect on free radical reduction. Propofol could be beneficial as an anesthetic in patients presenting pathologies associated with free radical reactions during CPB. The potent antioxidant potential of propofol during cardiopulmonary bypass (CPB) in adults was investigated. The selected 30 patients received open heart surgery under CPB were randomly divided into groups A and group B. The patients in the group A and group B were given propofol ( 0.1 mg.kg -1 .min -1) and fentanyl (5 μg.kg -1 .min -1) respectively to maintain anesthesia after aorta was cross-clamped. Blood samples were drawn pre anesthesia, pre CPB, at 30 min of CPB , at the end of CPB, at 1 h after CPB, at the end of operation, at 12 and 24 h postoperatively. RBC suspension was prepared and erythrocyte glucose 6 phosphate dehydrogenase (G 6 PD) and phosphofructokinase (PFK) activities, total erythrocyte In the group A, G 6 PD and PFK activities and GSH / GSSG ratio were almost uneventfully during CPB and postoperatively. In the group B, G 6 PD activity was in creased and PFK activity and GSH / GSSG ratio decreased significantly from 30 min of CPB until 12 h postoperatively. It was demonstrated that propofol could be attenuated free radical activity during CPB, while fentanyl has no effect on free radical reduction. Propofol could be beneficial as an anesthetic in patients presenting pathologies associated with free radical reactions during CPB.