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To investigate the influence of hypertension and ageing on the in vivo effect of acetylcholine on blood pressure and regional vascular conductance and the inhibitory effect of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide synthesis, on the effects of acetylcholine.Male spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats aged 3–4, 7–8 and 20–26 months, were anaesthetized with pentobarbital. After sino-aortic denervation, regional blood flow in the left common carotid, superior mesenteric, right renal and right femoral arteries was measured with pulsed Doppler flowmeters. After pretreatment with indomethacin (2 mg/kg intravenously), bolus injections of acetylcholine (0.03, 0.1 and 0.3 μg/kg intravenously) were given with or without L-NMMA (20 mg/kg intravenously).Dose-dependent hypotensive responses to acetylcholine were significantly smaller in SHR than in WKY rats in each age group. Ageing reduced the hypotensive effect of acetylcholine in both SHR and WKY rats. In rats aged 3–4 and 7–8 months acetylcholine-induced increases in regional vascular conductances were significantly smaller in SHR than in WKY rats. Ageing reduced acetylcholine-induced increases in regional vascular conductances in WKY rats. L-NMMA had little effect on the responses to acetylcholine in both SHR and WKY rats. Sodium nitroprusside had similar effects in SHR and WKY rats and in age groups within each strain.The in vivo vasodilatory action of acetylcholine, which appeared to be partly independent of nitric oxide, was impaired by hypertension and age.