This investigation was undertaken to characterize the postsynaptic alpha-adrenoceptors in isolated canine mesenteric arterial and venous preparations. Contractile responses to cumulative additions of phenylephrine (selective alpha 1-adrenoceptor agonist), UK-14,304 (selective alpha 2-adrenoceptor agonist), noradrenaline (non-selective alpha-adrenoceptor agonist), and dopamine (non-selective alpha-adrenoceptor agonist) were measured in the presence and absence of rauwolscine, a selective alpha 2-antagonist, and terazosin, a selective alpha 1-antagonist. Phenylephrine was a more potent agonist in the mesenteric artery than in the mesenteric vein; UK-14,304 exhibited the opposite profile of activity. Terazosin was a more potent antagonist than rauwolscine against each of the agonists, except dopamine, in the mesenteric artery but rauwolscine was more potent than terazosin in the vein. Terazosin and rauwolscine were equipotent in inhibiting the contractile responses to dopamine in the artery while rauwolscine was more potent than terazosin in the vein. The pA2 values measured in both vessels failed, however, to demonstrate a high selectivity for either alpha-adrenoceptor antagonist. These results suggest that the alpha-adrenoceptors in the canine mesenteric artery and vein exhibit pharmacological characteristic typical of both alpha 1- and alpha 2-adrenoceptor subtypes.