To determine the efficacy of MCI-186, a novel free radical scavenger, in ischemia-reperfusion injury during cardiopulumonary bypass (CPB), the following experimental model was applied. Twenty-one mongrel dogs were subjected to 120 minutes of hypothermic global ischemia by aortic cross clamping with intermittent administration of a cardioplegic solution followed by 60 minutes reperfusion. They were assingend to three groups : group A(n=7), no drug was administerd before reperfusion ; group B(n=7), 1ml/kg of physiological saline was administered by bolus injection through the aortic root just before reprefusion ; group C(n=7), MCI-186 (3mg/ml/kg) dissolved in physiological saline was administered in the same manner as group B. Cardiac function (left ventricular systolic pressure, cardiac output, left ventricular maximum dP/dt) after 60 minutes reperfusion expressed as as percent recovery of pre-ischemic state was superior in group C than in groups A and B. Release of lipidperoxide assayed by the TBA method as the difference between coronary artery and sinus were suppressed in the early phase of reperfusion in group C. Myocardial water content after 60 minutes reperfusion was also less in group C than in group A and B. These findings suggest that administration of MCI-186 before reprefusion after ischemia is effective in protecting the heart from ischemia-reperfusion injury.