We tested the hypothesis that arterial hyperoxia during myocardial reperfusion increases reperfusion injury and infarct size. The anterolateral marginal coronary artery of 35 anesthetized rabbits was occluded for 45 min, then reperfused for 3 h with either normoxic [arterial PO2 (PaO2) = 96.7 +/- 22.9 mmHg)] or hyperoxic (PaO2 = 554.8 +/- 61.7 mmHg) blood. In the hyperoxic group only, PaO2 was adjusted 10 s before the onset of reperfusion by raising inspired oxygen concentration to 100%. The area of infarction (AI) was defined by triphenyltetrazolium staining, and the area at risk (AR) by fluorescent microspheres. These areas were measured by planimetry. Heart rates and blood pressures did not differ between the two groups during occlusion or reperfusion. Infarct size (AI/AR) was 49.1 +/- 16.5% in the normoxic group (n = 17) and 40.8 +/- 16.1% in the hyperoxic group (n = 18). From these data, 90% confidence limits establish that the maximal true increase in AI/AR caused by hyperoxia would be 0%-1%. Hyperoxic reperfusion of ischemic myocardium compared with normoxic reperfusion does not significantly increase myocardial infarct size.