The blood plasmas of rats and dogs which had been subjected to 1-2 hrs. of hemorrhagic hypotension contained a pressor principle capable of causing a sustained elevation of blood pressure when injd. intraven. into cats, dogs and rats which had been nephrectomized 1-3 days before. The blood plasmas of human beings who had died following a p''ro-longed period of hypotension possessed similar sustained pressor activity, as detd. by injn. into nephrectomized cats, but almost imperceptible activity in nephrectomized rats. When semicrude extracts of horse, sheep, hog, cat, dog, rat and rabbit kidneys were injd. intraven. into nephrectomized cats, dogs, rats and non-nephrectomized chickens, the animals'' blood plasma acquired the ability to cause a sustained elevation of blood pressure in nephrectomized cats, dogs and rats, but not in non-nephrectomized chickens. The plasmas of chickens and cats which were injd. with chicken kidney extracts did not cause sustained pressor responses in the nephrectomized cat, dog or rat. Human kidney extracts, when injected into nephrectomized cats, dogs and rats, caused the appearance in the blood plasma of a sustained pressor principle which was active in cats and dogs but only very slightly active in rats. The specificity characteristics of the sustained pressor principle are similar to those of renin. Th''e principle may be recovered without appreciable loss from nephrectomized animals which have been given large quantities intraven. 1-2 hrs. before. It is concluded that the sustained pressor response which follows the injection of an active plasma is probably due to the continuous circulation of an undiminishing quantity of the sustained pressor principle in the blood stream.