Myocardial ischemia and reperfusion (MI/R) initiates a cascade of polymorphonuclear neutrophil (PMN)-mediated injury, the magnitude of which may be influenced by the bioavailability of nitric oxide (NO). We investigated the role of endothelial cell nitric oxide synthase (ecNOS) in MI/R injury by subjecting wild-type and ecNOS-deficient (−/−) mice to 20 min of coronary artery occlusion and 120 min of reperfusion. Myocardial infarct size represented 20.9 ± 2.9% of the ischemic zone in wild-type mice, whereas the ecNOS −/− mice had significantly (P < 0.01) larger infarcts measuring 46.0 ± 3.8% of the ischemic zone. Because P-selectin is thought to be involved with the pathogenesis of neutrophil-mediated I/R injury, we assessed the effects of MI/R on P-selectin expression in the myocardium of wild-type and ecNOS −/− mice. P-selectin expression measured with a radiolabeled monoclonal antibody (MAb) technique after MI/R in wild-type mice was 0.037 ± 0.009 μg MAb/g tissue, whereas ecNOS −/− coronary vasculature was characterized by significantly (P < 0.05) higher P-selectin expression (0.080 ± 0.013 μg MAb/g tissue). Histological examination of the postischemic myocardium revealed significantly (P < 0.01) more neutrophils in the ecNOS −/− (29.5 ± 2.5 PMN/field) compared with wild-type (5.0 ± 0.9 PMN/field) mice. A similar trend in infarct size and neutrophil accumulation was observed when wild-type and ecNOS −/− mice were subjected to 30 min of ischemia and 120 min of reperfusion. These novel in vivo findings demonstrate a cardioprotective role for ecNOS-derived NO in the ischemic-reperfused mouse heart.