Objective: Both disruption of the endothelial nitric oxide synthase (eNOS) gene and pharmacological inhibition of the NOS produce modest hypertension. It is unclear if and to what extent NOS isoforms other than eNOS contribute to this effect and how loss of one copy of the eNOS gene might impact on vascular reactivity or eNOS protein expression. Methods: We examined protein expression, vascular reactivity, activity of soluble guanylate cyclase, blood pressure and heart rate in mice completely lacking the eNOS gene (eNOS_−/−), wild-type mice (eNOS_+/+) and mice heterozygotic for the eNOS gene (eNOS_+/−). Results: While eNOS_−/− mice had mild hypertension and bradycardia, eNOS_+/− mice were normotensive. In control mice, oral administration of l-NAME (approximately 100 mg/kg/day×21 days) increased blood pressure to levels observed in eNOS_−/− mice. In eNOS_−/− mice, chronic oral administration of l-NAME had no effect on blood pressure, suggesting that inhibition of other NOS isoforms unlikely contribute to hypertension. l-NAME treatment induced bradycardia in both control and eNOS_−/− mice, suggesting that both eNOS and other isoforms of NOS might be involved in heart rate control. Studies of aortic rings from eNOS_−/− mice revealed a complete lack of endothelium-dependent vascular relaxation in response to acetylcholine and the calcium ionophore A23187 and an increase in sensitivity to phenylephrine, serotonin and nitroglycerin. Aortic rings from eNOS_+/− mice demonstrated only minor alterations of responses to nitroglycerin and a normal relaxation to either acetylcholine or A23187 compared to vessels from eNOS_+/+. Western analysis demonstrated that eNOS expression was virtually identical between eNOS_+/+ and eNOS_+/− mice and was absent in eNOS_−/− mice. The activity of lung-isolated soluble guanylate cyclase was identical in the three strains of mice. Conclusions: We conclude that loss of one copy of the eNOS gene, as observed in heterozygotic animals, has no effect on vascular reactivity, blood pressure or eNOS protein expression. Isoforms of NOS, other than eNOS are unlikely involved in blood pressure regulation but may participate in heart rate control.