Free- radicals are generated in the minutes following myocardial re-perfusion and this could be the cause of myocardial injury during by Pass surgery. Pretreatment with antioxidants could effectively attenuate re-perfusion injury during global ischemia.Xanthine dehydrogenase is converted into xanthine oxidase and this causes accumulation of hypoxanthine and xanthine in the cell. When re-perfusion starts, fresh oxygen interacts with xanthine oxidane and hypoxanthine to form superoxide anion radical O2.- . Allopurinol can metabolically block the formation of reduced oxygen radical by inhibiting the enzyme xanthine oxidase. More over allopurinal has vasodilator effects and aids in restoring favorable conditions through enhancement of collateral blood flow to the ischemic area.