Many Investigations have evaluated the effect of oxygen free radical scavenger on the infarct size after reperfusion. but those were not consistent in results. Many hypotheses for the discrepancy were proposed but not satisfactory. The present study evaluated the ability of superoxide dismutase-polyethylene glycol(SOD-PEG) to reduce the infarct size,and the effect of cell swelling on the result, and parameters influencing the infarct size. Coronary artery was occluded for 45 minutes and reperfused for 4 hour.In 5 rabbits,SOD-PEG was infused for 15 minutes from 15 minutes before reperfused. In 7 rabbit, 5% dextrose solution was administrated instead. Infarct size was measured by triphenyl terazolium chloride and risk area by monastral blue. Water content was calculated from difference of weights before and after drying in 85degrees C for 24 hours. Blood pressure, heart rate, and double product were analysed. The results are as follows; 1) hemodynamic parameters of blood pressure, heart rate, and double product were not different between two groups. The risk volume (p>0.05) and infarct volume(p<0.05) were smaller in treated group than in control group, and the ratio of risk area to total myocardium was lower in treated group than in cotrol group without statistic significance. 2) The percentage of infarct size in risk area was smaller in treated group than control group(p<0.05) 3) There was no difference in water content of normal, risk, and infarct areas. 4) Blood pressure,heart rate,and double product were not related to the infarct size directly. 5) The ratio infarct size to risk area had good positive relation with the ratio of risk area to total myocardium in total(R=0.84, p<0.001) and control groups(R=0.89, p<0.01). In conclusion, SOD-PEG could not rule out the infarct size and cell swelling didn't infulence area in treated group although the ratio of risk volume to total was not different statisitically between two groups.