Summary.Alcoholic extracts of whole blood from cattle and normal human subjects produce a rise of blood pressure and tachycardia in the cat, after adsorption of depressor constituents with Fuller's earth. The active substance is inactivated by iodine in neutral or slightly alkaline solutions.The pressor action is enhanced by cocaine, and inhibited or abolished but not reversed by a dose of dihydro‐ergotamine sufficient to reverse the action of an equipressor amount of adrenaline.The evidence presented supports the assumption that the normally occurring pressor agent in bovine and human blood is chiefly nor‐adrenaline (sympathin).The amount of sympathin found in bovine and human whole blood corresponds to some 2–4 μg dl‐nor‐adrenaline hydrochloride per 100 ml blood.This work has been supported by a grant from the Rockefeller Foundation to one of us (U. S. v. E.) which is gratefully ac