Previous studies indicate that deficiency of endothelial nitric oxide (NO) synthase (eNOS)-derived NO exacerbates myocardial reperfusion injury. We hypothesized that overexpression of eNOS would reduce the extent of myocardial ischemia-reperfusion (MI/R) injury. We investigated two distinct strains of transgenic (TG) mice overexpressing the eNOS gene (eNOS TG). Bovine eNOS was overexpressed in one strain (eNOS TG-Kobe), whereas the human eNOS gene was overexpressed in the other strain (eNOS TG-RT). Non-TG (NTG) and eNOS TG mice were subjected to 30 min of coronary artery occlusion followed by 24 h of reperfusion, and the extent of myocardial infarction was determined. Myocardial infarct size was reduced by 33% in the eNOS TG-Kobe strain (P< 0.05 vs. NTG) and by 32% in the eNOS TG-RT strain (P< 0.05 vs. NTG). However, postischemic cardiac function (cardiac output, fractional shortening) was not improved in the eNOS TG-Kobe mouse at 24 h of reperfusion [P= not significant (NS) vs. NTG]. In additional studies, eNOS TG-Kobe mice were subjected to 30 min of myocardial infarction and 7 days of reperfusion. Fractional shortening and the first derivative of left ventricular pressure were measured in eNOS TG-Kobe and NTG mice, and no significant differences in contractility were observed (P= NS) between the eNOS TG mice and NTG controls. Left ventricular end-diastolic pressure was significantly (P< 0.05 vs. NTG) reduced in the eNOS TG-Kobe strain at 7 days of reperfusion. The cardioprotective effects of eNOS overexpression on myocardial infarct size were ablated by N ω-nitro-l-arginine methyl ester (300 mg/kg) pretreatment. Thus genetic overexpression of eNOS in mice attenuates myocardial infarction after MI/R but fails to significantly protect against postischemic myocardial contractile dysfunction in mice.