Results obtained in a series of experiments by one of the writers on the factors involved in the metamorphosis of the frog tadpole clearly indicated that the physiologic responses of amphibians and mammals to thyroid administration depend upon different chemical groupings within the hormone (Swingle, '22, '23, '24). The essential fact brought to light in these experiments which has a bearing on the present work, was that substances which exert a thyroid-like effect when administered to amphibians may be without demonstrable effect upon mammals, the criterion for thyroid activity in amphibians being the metamorphic changes induced, and in mammals, the effect upon the basal metabolic rate. F o r example, certain iodized proteins, such as iodocasein, iodoserum globulin, iodoserum albumen, iodofibrin, and the amino-acid iodotyrosine, when fed or injected into thyroidectomized or hypophysectomized frog larvae, quickly induce metamorphosis, thus simulating a thyroid-like activity. These same substances are without effect upon amphibian transformation without iodine in the molecule, nor can other halogens, such as bromine, be substituted for the iodine, and the substances still retain their activity. Numerous workers have established ' P a r t of the expenses of this investigation was defrayed by a grant from the Bache Fund of the National Academy of Sciences.