We tested whether recombinant human superoxide dismutase conjugated to polyethylene glycol (PEG-SOD) to prolong its plasma retention time could limit myocardial infarct size in an ischemia-reperfusion model in the rabbit. One group of animals received 1000 units/kg of PEG-SOD as an intravenous bolus 15 min before coronary occlusion. A second group received saline only and served as controls. Under pentobarbital anesthesia, a left coronary branch was occluded for 30 min and then reperfused. The surgical wounds were repaired and the animals were allowed to recover. Seventy-two hours after the coronary occlusion, the heart was excised and the size of the area at risk (ischemic vascular bed) was assessed with fluorescent particles and the infarct size was determined by histology (Hematoxylin-eosin, Azan stain). Infarct size as a percentage of the area at risk was similar between the groups, 46.5 + 2.7 in the PEG-SOD group (n = 8) and 48.9 + 3.1 in the control group (n = 8). There were no significant differences between the groups indicating that PEG-SOD did not limit infarct size in this model.