During the past few months I have been engaged in a study of the physiologic action of the thyroid. The general scheme followed has been to treat dogs with a continuous injection1of amino acids, sugars and other substances in conjunction with the injection of the active constitutent of the thryroid, and to study the effect of the thyroid on the metabolism of the animals. Some animals respond vigorously, the injection of the thryoid constitutent producing very high pulse rate and a great increase in the respiration, accompanied by an extremely severe general tetany and terminating in a temperature which occasionally reaches as high as 112. It was soon found that all animals can be divided into two classes, those which give this reaction and those which do not. For certain reasons it seemed probable that the condition of the animal could be foretold by the use of some substance