The efficacy of recombinant human superoxide dismutase (h-SOD) was examined and its optimal dose when given before reperfusion in an experimental canine cardiopulmonary bypass (CPB) model was determined. Mongrel dogs were placed on total CPB for 130 minutes without aortic cross clamping (Group Ⅰ). Others were placed on CPB for 120 minutes aortic cross clamping with intermittent administration of cardioplegic solution and core cooling (Group Ⅱ). Before reperfusion, saline, and 1 mg, 3 mg, 10 mg and 20 mg h-SOD per kilogram were administered via the aortic root as a bolus injection (Group Ⅲ,Ⅳ,Ⅴ,Ⅵ,Ⅶ). Reperfusion after hypothermic global ischemia with aortic cross clamping deteriorated cardiac function (cardiac index, left ventricular maximum dp/dt), increased myocardial water content and increased cardiac enzyme release (creatinine kinase MB isozyme, α-hydroxybutyric dehydrogenase). Administration of 3 mg/㎏ h-SOD significantly ameliorated this reperfusion injury, protected myocardial function early after CPB and gave a desirable peak serum h-SOD concentration.