We studied isolated canine arteries and veins to compare the pharmacological properties of their postjunctional alpha-adrenoceptors. Rings of femoral and splenic arteries and of femoral and saphenous veins were mounted for isometric tension recording in organ chambers filled with Krebs-Ringer bicarbonate solution. The four blood vessels contracted when exposed to methoxamine, norepinephrine, phenylephrine, and tramazoline; clonidine failed to induce contraction only in the splenic artery. The relative sensitivity to methoxamine was comparable in the arteries and veins, but that for phenylephrine was larger in the former. The veins, but not the arteries, were more sensitive to clonidine and tramazoline than to phenylephrine or methoxamine. Phentolamine was a competitive antagonist against norepinephrine in the arteries and the veins. Prazosin was a competitive antagonist arteries. The competitive antagonistic properties of yohimbine were more pronounced in the veins than in the arteries. Verapamil depressed to the same extent the contractile responses of saphenous veins to clonidine and norepinephrine, but reduced the contractions caused by methoxamine more than those due to norepinephrine. These results indicate the presence of both alpha 1- and alpha 2-like adrenoceptors on venous smooth muscle cells, whereas arterial smooth muscle cells contain mainly postjunctional alpha 1-adrenoceptors.