Experimental evidence indicates that an increased production of nitric oxide could play a role in the peripheral vasodilation of portal hypertension. To test this hypothesis in humans, we studied basal serum NO(2-) + NO3- levels and the response of forearm resistance vessels to increasing concentrations of methacholine chloride, sodium nitroprusside, and phenylephrine infused into the brachial artery of 12 cirrhotic patients and 10 controls. Forearm vascular resistance (FVR) was calculated from mean arterial pressure and forearm blood flow (FBF). Cirrhotics showed higher NO(2-) + NO3- levels (P < 0.05), higher FBF (P < 0.01), and lower FVR (P < 0.01) than controls. The reduction of FVR in response to every dose of methacholine was greater in cirrhotics than in controls; this was significant (P < 0.05) at the 3 and 10 micrograms/min doses. This response to methacholine was not modified by blockade of vascular prostacyclin. The response to nitroprusside was similar in both groups. The increase in FVR in response to every dose of phenylephrine was significantly (P < 0.01) lower in cirrhotics than in controls. In cirrhotics, a significant correlation (r = -0.81, P < 0.01) was found between the FVR response to the highest doses of methacholine and phenylephrine. In conclusion, cirrhotic patients show an enhanced endothelium-mediated vasodilation, which suggests an increased synthesis of nitric oxide. This defect may mediate the peripheral vasodilation and hyporeactivity to vasopressors of these patients.