Several in vitro studies have shown that endothelium-dependent vasodilatation is maintained by endothelium-derived hyperpolarizing factor (EDHF) or prostacyclin in vessels isolated from endothelial nitric oxide synthase knockout mice. Since this has not been addressed by in vivo studies, we sought to define the magnitude and the onset time of this compensation by recording blood pressure responses to endothelium-dependent vasodilators in rats treated acutely or chronically with the NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME). Groups of male Sprague-Dawley rats were given plain water (control) or L-NAME (0.7 mg/ml) in drinking water for 1 day, 5 days, 3 wks or 6 wks. Dose-dependent hypotensive responses to acetylcholine, bradykinin and sodium nitroprusside were determined in anesthetized rats before and after acute intravenous infusion of either L-NAME or a combination of apamin plus charybdotoxin that would selectively inhibit EDHF. Acute L-NAME treatment increased the mean arterial pressure and inhibited acetylcholine- and bradykinin-induced fall in blood pressure in control but not in chronic L-NAME treated rats. The endothelium-dependent hypotensive responses to acetylcholine and bradykinin were restored in rats treated with L-NAME after a time period of 24 h along with increased sensitivity to sodium nitroprusside and reduced plasma nitrate+nitrite levels. While apamin+charybdotoxin pretreatment inhibited the responses to acetylcholine and bradykinin in both acute and chronic L-NAME treated groups, it was more pronounced in the latter group. In conclusion, chronic inhibition of nitric oxide synthase results in the development of a compensatory hypotensive response to acetylcholine within 24 h and this is mediated by EDHF.