We investigated if superoxide dismutase and catalase could reduce myocardial infarct size in an open chest occlusion-reperfusion model. Thirty pigs were used for the experiment. The left anterior descending artery was ligated for 60 min followed by a 5 h reperfusion period. After randomisation and blinding the two enzymes or placebo were injected into the left atrium as a bolus immediately before and at the end of the occlusion and as a continuous infusion over the first hour of the reperfusion period. The total dose for each enzyme was 8 mg/kg bw. Tetrazolium staining was used to determine infarct size. The study code was not broken until all calculations and exclusions had been made. Nine animals died from intractable ventricular fibrillation, most commonly during the occlusion. Another three were excluded for technical reasons. We found that superoxide dismutase and catalase reduced infarct size in relation to myocardium at risk from a mean of 89% to 63% (P less than 0.01). Initial plasma half life for the two enzymes after the bolus infusions were calculated to be 30 min.