High melanophore hormone fractions (I, II, III) were obtained which caused a marked pigment dispersion for 24 hrs. after the inj. of 1 cc. of a 1 X 10-6 dilution into the hypophysectomized frog. By definition these fractions were referred to as high melanophore hormone fractions because they gave a marked physiological response in hypophysectomized frogs when injected in high dilutions. Fractions I and II produced a marked acceleration of the oxidase system tyrosine-tyrosinase. Fraction III which was prepared from the ant. lobe of the finback and the white whale pituitary caused a marked decrease (140%) in dopa autoxidation, as well as tyrosine-tyrosinase oxidation (22%). The possibility of the formation of this inhibitor during chemical manipulation must be considered. The pressor hormone and, perhaps, the oxytocic hormone inhibited the oxidase system tyrosine-tyrosinase. However, a fraction was obtained from the ant. pituitary lobe which also caused an inhibition of tyrosine-tyrosinase oxidation. It might be this factor, present in both lobes, which inhibits the oxidative process, as well as the pressor and oxytocic hormones. Liver, kidney, lung and muscle extracts had no effect on the tyrosine-tyrosinase system. These same extracts had much less inhibitory effect on dopa oxidation than did the pituitary fractions.