AN increase in blood pressure is produced by compression of the renal artery1 or by injection of the venous blood of the kidneys2. The filtrate obtained after adding 3 vol. of acetone to the serum of this blood contains a pressor substance which is insoluble in ether and amyl alcohol, soluble in glacial acetic acid and is destroyed only after three hours boiling in normal hydrochloric acid. The same substance is formed on incubating for fifteen minutes at 37° the kidney protein renin3 with blood serum or its pseudo-globulin fraction. This substance, which we name hypertensin, is different from adrenalin, tyramin, pitressin and urohypertensin. Renin appears to be a proteolitic enzyme of the papain type, which liberates hypertensin from a blood protein belonging to the pseudo-globulin fraction.