A previous study by the authors showed that myocardial infarct size in the rabbit, measured after 45 mins of ischemia and 3 h of reperfusion, could be limited by administration of superoxide dismutase (SOD) plus catalase. The present study examined whether this infarct size limitation is sustained for the following three days. Under anesthesia, a coronary branch of the Japanese white rabbit was occluded for 45 mins and then reperfused. Three days after surgery, the heart was excised and the volume of myocardium supplied by the occluded coronary branch (ischemic zone size) was assessed with fluorescent particles and the infarct size was estimated by hematoxylin and eosin and with Mallory's staining. The SOD plus catalase group (n = 14) received 15,000 units/kg of SOD plus 50,000 units/kg of catalase in saline over 90 mins, starting 15 mins before the coronary occlusion. Saline was infused in the control group (n = 15). Three rabbits in the control group and three in the SOD plus catalase group died of ventricular fibrillation during the ischemic period. Three control and two SOD plus catalase rabbits were excluded because the ischemic zone was ambiguous. The percentage of the ischemic zone which was infarcted was 59.4 +/- 6.9% (mean +/- SE) in the control group (n = 9) and 49.4 +/- 5.1% in the SOD plus catalase group (n = 9). These were not statistically different. Ischemic zone size and hemodynamic parameters were similar in the two groups. These findings suggest that SOD plus catalase may serve only to delay rather than prevent myocardial infarction.