Pharmacological effects of lysine and arginine vasopressins are the same with respect to their relative vasopressor, milk-ejecting and avian depressor potencies. The ratio of pressor potency to antidiuretic potency is 1 for arginine vasopressin and 6 for lysine vasopressin when the antidiuretic potency is determined by intravenous injection in the hydrated, unanesthetized dog. On the other hand, this ratio of potencies is 1 for both vasopressins when the antidiuretic potency is determined by subcutaneous injection in the hydrated, unanesthetized rat. Lysine vasopressin, compared with the U.S. P. standard or arginine vasopressin, is equally potent as an intravenous pressor agent in the dog and rat. Potency refers to relative potency; potency in units per mg is much higher for purified arginine vasopressin. Lysine vasopressin is peculiar to the hog. The vasopressin of man, the macaque monkey, dog, rat, ox, sheep and camel appears, by pharmacological criteria, to be arginine vasopressin. These conclusions are tentative and must be confirmed by the isolation of the hormone from each species, other than the ox and hog, together with the demonstration of the amino acid composition of each vasopressin.