It has been suggested that increased production of nitric oxide by an inducible nitric oxide synthase isoenzyme is important in the pathogenesis of the vascular abnormalities seen in human beings and animals with portal hypertension. We investigated this hypothesis by studying the in vitro vascular reactivity of isolated aortic rings from portal vein—constricted and sham-operated rats. Aortic rings from portal vein—constricted rats exhibited significantly impaired contractility to phenylephrine and potassium chloride compared with control rats. Preincubation with the nitric oxide synthase inhibitor nitro-l-arginine methyl ester significantly increased contractility to phenylephrine and potassium chloride in both portal-hypertensive and control tissues, with greater effect in the portal-hypertensive rings. Despite nitro-l-arginine methyl ester, maximal contractions were still significantly smaller in the portal-hypertensive tissues. Vascular relaxation evoked by acetylcholine, but not by the endothelium-independent vasodilator glyceryltrinitrate, was significantly impaired in the portalhypertensive group. Our results demonstrate significant impairment in vascular function in aortic rings in this model of portal hypertension. The addition of a nitric oxide synthase inhibitor partly corrected these changes, suggesting that although nitric oxide is likely an important mediator, other factors may also be involved in the pathogenesis of these alterations in vascular function. (Hepatology 1994;20:1516-1521).