The anterior mesenteric vein of the rabbit responds to substance P with dose-dependent contractions and is among the vascular smooth muscles most sensitive to this peptide. In spite of its high sensitivity to numerous other agents, including angiotensin and bradykinin, the rabbit mesenteric vein can be made selective for substance P by the use of specific inhibitors that will prevent the myotropic effects of acetylcholine, catecholamines, histamine, 5-hydroxytryptamine, and of the two above-mentioned peptides, without modifying the contractions elicited by substance P. It appears that this peptide acts directly on specific receptors and not through the release of neurotransmitters. Interference by intramural prostaglandins is excluded because substance P is equally active on tissues pretreated with indomethacin or untreated. Dose-response curves obtained with substance P are close to the theoretical curves predicted by the mass action law. The rabbit mesenteric vein contains a new type of receptor for bradykinin, recently identified (REGOLI, D., MARCEAU, F., and BARABE, J. 1978. De novo formation of vascular receptors for bradykinin. Can, J. Physiol. Pharmacol. 56, in press.). The action of bradykinin on this receptor can be prevented with the use of specific and competitive inhibitors and, therefore, the mesenteric vein will distinguish between peptides of the kinins or of the substance P types.