Arterial contractility was investigated, using spiral and ring sections of dog, cat, rabbit, rat, and human vessels. Epinephrine produced contractions of all arterial tissue tested. Histamine invariably produced contractions. The phenomenon of variable sensitivity of arterial tissue to histamine was again demonstrated. Acetylcholine caused contractions of rabbit, cat, and rat aortas, but no contraction of the dog aorta, or dog and human femoral arteries was observed. Pitressin produced contraction of the rat aorta, and dog and human femoral arteries, but not of rabbit, dog or cat aortas.Using dog vessels, a difference in activity was observed between the aorta and the femoral artery. Greater reactivity in terms of maximum rate of tension increase and tension developed was seen in the smaller systemic arteries. A pharmacologic difference with respect to the action of pitressin was discovered between the aorta and the smaller arteries. Thus, general evidence is presented suggesting that data obtained in 1 species of animal on 1 type of tissue cannot be transferred without specific proof of its validity to data obtained in other species, or even in closely related tissues of the same species.An apparent antagonistic effect of pitressin on the pressor action of epinephrine was detected.