The effect of ifenprodil tartrate (IFT) on 2 subtypes of postsynaptic alpha 1- and alpha 2-adrenoceptors was investigated. For this purpose, changes in the tension of isolated canine saphenous veins were measured isometrically, and Ca2+ uptake in these veins was also measured. IFT displaced the dose-contractile response curves for phenylephrine and clonidine in a competitive manner, and the activity sequence of antagonists against phenylephrine was IFT greater than phentolamine greater than prazosin greater than yohimbine. On the other hand, the activity sequence of antagonists against clonidine was IFT greater than phentolamine greater than yohimbine greater than prazosin. The effect of IFT as well as that of phentolamine were different from the other antagonists and of approximately the same specific activities against both agonists. The IFT-induced relaxations of the veins contracted by phenylephrine and clonidine were relatively unchanged in both normal and Ca2+-free solutions. IFT inhibited phenylephrine- and clonidine-induced Ca2+ uptake in the saphenous veins. The results suggest that IFT does not possess a relative selectivity toward postsynaptic alpha 1- and alpha 2-adrenoceptors, and that these actions reflect the vasorelaxing effects on phenylephrine- and clonidine-induced contraction in the canine saphenous veins.