Increased levels of circulating vasodilators have been claimed to be the causative factor in the hyporesponsiveness to endogenous vasopressors in portal hypertension. To investigate whether this hyporeactivity to vasopressors is also present in an in vitro system perfused with a synthetic medium, the responsiveness to graded concentrations of norepinephrine, arginine-vasopressin, and potassium chloride was tested in perfused superior mesenteric arterial beds of normal rats and rats with portal hypertension induced by partial portal vein ligation (PVL). The same vasopressors were tested after incubation of vessel preparations with the stereospecific nitric oxide formation inhibitor N^ω-nitro-l-arginine (NNA, 10⁻⁴ mol/L). Vessel preparations of PVL compared with normal rats (n = 8 per group and vasopressor) expressed a significant (P < 0.05) hyporeactivity to norepinephrine, arginine-vasopressin, and potassium chloride over a wide range of concentrations. This hyporesponsiveness was overcome by preincubating vessel preparations with NNA. In summary, portal hypertension is accompanied by a significant in vitro hyporeactivity of splanchnic vessels to norepinephrine, argininevasopressin, and potassium chloride, and secretion of nitric oxide in this preparation seems responsible for this blunted response.