The superior mesenteric blood flow response to intra-arterial injections (0.5-25 .mu.g) and infusions (5-30 .mu.g/min) of 5-hydroxytryptamine (5-HT, serotonin) was investigated in anesthetized cats in which nerve activity to the intestine was altered by surgical and pharmacological procedures. With the superior mesenteric periarterial nerves intact, low doses of 5-HT (< 5 .mu.g) produce vasodilatation, whereas higher doses produce vasoconstriction. When the periarterial nerves are cut either at the start of or during the experiments, vasodilatation is elicited over the entire dose range, and doses of 5-HT which initially produce vasoconstriction elicit vasodilatation after nerve sectioning and also after alpha-adrenergic receptor blockade. These vascular responses are not secondary to changes in arterial pressure or intestinal motility. The vasodilator response to 5-HT is unaffected by .alpha.- or .beta.-adrenergic or cholinergic receptor blockade, by ganglionic blockade, or by histamine receptor blockade, but is blocked by tetrodotoxin and also the 5-HT antagonist, dihydroergotamine.