In order to investigate oxygen free radical scavenging effect of the ascorbic acid (vitamin C), two groups of patients were formed in whom valve surgery planned. In the study group (Group I) high doses of ascorbic acid (15 gr) were administered via IV route during induction of anesthesia. Control group (Group II) did not receive ascorbic acid. During cardiopulmonary bypass systemic hypothermia (28°C), topical hypothermia, and cold intermittant crystalloid cardioplegia (10 ml/kg initially, then 5 ml/kg repeated every 30 minutes) was applied. Mean aortic cross clamp times were similar (Group I:71.8±3.7 min, Group II:73.5±5.2 min, p<0.05) and never less than 30 minutes. Blood samples for biochemical study (lactate, CPK-MB, uric acid, glutathion, vitamin E, malondialdehyde measurements) were taken 1 min, 5 min, 15 min after declamping of the aorta and 15 min after the termination of cardiopulmonary bypass. Hemodynamic measurements of patients (heart rate, cardiac index, systemic vascular resistance, left ventricular stroke work index) were recorded before cardiopulmonary bypass, 15 min, 2 hr, 4 hr, 6 hr, 9 hr, 12 hr, 24 hr after termination of cardiopulmonery bypass. Results of biochemical analysis supported the effect of ascorbic acid as an oxygen difference between two groups. This is tought to be related to the factors which were non uniformly distributed in both groups (pulmonary hypertension, positive inotropic, preload, afterload reducing medications) directly effecting the hemodynamic condition.