Previously Dr. Meek and I1 made a comparative study of ephedrine, tyramine and epinephrine, with special reference to the circulation, and made the assertion that the stability of ephedrine to physical and chemical agents, and that its persistence of systemic action when taken by mouth, are chiefly caused by the absence of the hydroxyl group in the benzene ring. A similar opinion is also shared by Kendall of the Mayo Clinic.2 If, however, this were the only factor, the same would be expected to hold for β-phenyl-ethylamine, C6H5,CH2,CH2,NH2, and an identical effect expected from pseudo-ephedrine, which has the same structural formula but an opposite optical rotation (Dextrorotatory)— both of them being devoid of phenolic hydroxyls. The present study is undertaken to test whether or not this assumption is true by comparing the mydriatic action of these three amines in animals and in man, and their pressor action in dogs and