OBJECTIVE: The aim of this study was to compare the response to NO-mediated vasodilators in vivo and in vitro during chronic NO synthase inhibition. METHODS: NG-Nitro-L-arginine-methyl ester (L-NAME, 0.4 g/l) or vehicle was administered in the drinking water for 6 weeks to male Wistar rats weighing 220-240 g. The effect of acetylcholine and sodium nitroprusside was examined in vivo, on systemic blood pressure and heart rate and in vitro, on the precontracted isolated mesenteric artery. The in vivo response to both vasodilators was examined in awake rats monitored by an indwelling catheter in the femoral artery. Isolated segments of the third-generation mesenteric artery were examined in vitro with a Mulvany dual myograph after precontraction with noradrenaline. RESULTS: In isolated mesenteric arteries obtained from rats chronically treated with L-NAME, the initial relaxant response to acetylcholine was significantly decreased whereas that to sodium nitroprusside was enhanced. A late acetylcholine-induced contractile response was present and abolished by indomethacin. In vivo, the hypotensive action of sodium nitroprusside was also enhanced in the L-NAME-treated rats. Acetylcholine reduced blood pressure in the L-NAME-treated hypertensive animals more than in normotensive controls, but less than in control rats infused intravenously with noradrenaline at a dose increasing their blood pressure to hypertensive levels. CONCLUSIONS: The NO-mediated vasodilation induced by acetylcholine is attenuated during chronic NO synthase inhibition, both in vivo and in vitro. The blunted hypotensive response to acetylcholine can be demonstrated only if blood pressure of control rats is acutely increased to hypertensive levels.