The protective effect of a large dose Vit. C on ischemic myocardium was studied on 10 patients who underwent open heart operation and received 250 mg/kg Vit. C before the start of ECC (Group II). Another 10 similar patients receiving no Vit. C served as a control (Group I). Blood samples were taken from the brachial artery at different time intervals for determination of CPK-MB and MDA in both groups. Considerable increases of serum CPK-MB and MDA in both group I and II were noted. The differences of CPK-MB and MDA concentration between Group I and II were highly significant from 2 to 12 hours postoperatively. The effect of Vit. C in the reduction of the release of CPK-MB and MDA after open heart operation was attributed to its action as an oxygen free radical scavenger, thus decreasing the peroxidation of the lipids present in the cell membrane.