In valve replacement surgery, human superoxide dismutase (h-SOD) was administered just before reperfusion, and the effect of h-SOD on reperfusion injury was studied. Release of cardiac enzymes (CPK-MB and HBD), increase of left ventricular volume, and recovery rate of cardiac function were measured. The release of the enzymes during 60 minutes after reperfusion in the group receiving h-SOD (3mg/kg) was less than that of the control (without h-SOD)group (p<0.01). Myocardial edema (volume increase) was also reduced by h-SOD (1mg/kg or 3mg/kg : p<0.01). Recovery of cardiac function after reperfusion of the h-SOD group was better than that of the control group. In conclusion, h-SOD can protect the myocardium from reperfusion injury and accelerate the rate of recovery of cardiac function after global ischemia.