Endothelial dysfunction ensuing inhibition of nitric oxide synthase (NOS) was investigated in male Sprague-Dawley rats given N ω -nitro-l-arginine methyl ester (l-NAME) in drinking water for 8 weeks. Age-matched rats served as controls. l-NAME-treated rats, as compared to control animals, showed: (1) a clear-cut increase in systolic blood pressure; (2) a consistent decrease of endothelial-cell NOS (eNOS) gene expression in aortic tissue; (3) a reduction of the relaxant activity of acetylcholine (ACh, from 10 - 1 0 to 10 - 4 M) on norepinephrine-precontracted aortic rings (reduction by 52+/-5%); (4) a marked decrease (-50%) of the basal release of 6-keto-prostaglandin F 1 α (6-keto-PGF 1 α ) from aortic rings. In l-NAME-treated rats, administration in the last 2 weeks of either the angiotensin-converting enzyme inhibitor enalapril (1 mg/kg/day) or the cognate drug quinapril (1 mg/kg/day) decreased systolic blood pressure levels, completely restored eNOS mRNA levels in aortic tissue, and allowed a consistent recovery of both the relaxant activity of ACh and the generation of 6-keto-PGF 1 α . No difference was present in the ability of the two angiotensin-converting enzyme inhibitors to reverse NAME-induced endothelial dysfunction. These findings indicate that l-NAME-induced hypertension in the rat relies on the marked impairment of the endothelial vasodilator function, with an ensuing contribution by a decreased production of prostacyclin by the endothelial cells. Angiotensin-converting enzyme inhibition by enalapril or quinapril was equally effective in improving endothelial vasodilator function, prostacyclin endothelial production and restoring aortic eNOS mRNA.