The characterization of the receptor(s) involved in the 5-hydroxytryptamine (5-HT)-induced contraction was studied in the cat middle cerebral artery. 5-HT agonists and antagonists were tested on isolated arterial segments and their vascular potency correlated with their affinity value for 5-HT receptor subtypes as defined in the literature. 5-Carboxamidotryptamine (5-CT) and RU 24969 (5-HT1A-1B agonists) were more potent and as potent as 5-HT, respectively. Agonists at the 5-HT1C-2 (alpha-methyl-5-HT) and 5-HT3 (2-methyl-5-HT) sites were significantly less potent than 5-HT. All these agents induced similar maximal effects. The selective 5-HT1A agonist, 8-hydroxy-2(di-n-propylamino)tetralin, was the least potent but elicited a more intense vasoconstriction. The 5-HT-induced contraction was potently inhibited by compounds with high affinity at the 5-HT2 site in an apparently noncompetitive manner. The order of potency was: pizotifen greater than ritanserin greater than or equal to dihydroergotamine greater than or equal to ketanserin greater than or equal to methysergide. The nonselective agent methiothepin inhibited strongly the vasoconstriction induced by both 5-HT and 5-CT although a decrease in the maximal responses was observed together with a shift of the dose-response curves. In contrast to 5-HT, the contraction induced by 5-CT was only slightly affected by micromolar concentrations of ketanserin. 5-HT1A-1B (propranolol and cyanopindolol), 5-HT1C (mesulergine) and 5-HT3 (MDL 7222 and GR 38032F) selective drugs were almost completely devoid of significant antagonistic activity. Thus, the pharmacology of the feline cerebrovascular receptor(s) shares agonist similarities with the 5-HT1B/5-HT1D receptor subtypes whereas its antagonist profile relates more to that of the 5-HT2 site described in mammalian brain. Although these results may suggest the presence of two receptors in this cerebrovascular smooth muscle, they do not exclude the possibility that a single 5-HT receptor, not yet described by radioligand binding techniques, mediates the contractions induced by 5-HT. A definitive classification of this site, however, appears premature and would require novel and more selective agents.