IT was thought desirable to amplify by other methods the results obtained with tadpoles(1) in the study of the effects of natural and synthetic thyroxine and of various intermediate and related compounds prepared by Dr C. R. Harington. For this purpose it was decided to measure their action in increasing the gaseous metabolism of mammals.In the early investigations of the effects of thyroid preparations, an account of which will be found in Grafe's monograph(2), no attempt was made to obtain quantitative results. Thyroxine was first isolated by Kendall(3) in 1914, and was found to produce on injection all the known effects of thyroid preparations. This discovery was followed by various attempts to devise a reliable method of measuring thyroid activity. Plummer and Boothby(4) studied the effect of thyroxine on the basal metabolism of 69 cases of myxcedema and came to the conclusion that each milligramme of thyroxine would produce a maximal increase of 2X8 p.c. of the normal rate and that this maximuim occurred about 8 days after the injection. The "average variation" from this figure was only 25 p.c., and the relation appears to hold for all doses between 5 and 15 mg.Various German writers, using smaller doses, observed comparatively larger effects, after the injection of thyroxine, which developed and passed away in a couple of days (5).For many purposes, and in particular for the study of substances with unknown properties, it is desirable to make experiments with animals other than man. Unfortunately, the work that has been done in this direction has not hitherto given such clear results as those obtained by Plummer and Boothby.Meta Kunde (6) has published an account of a careful investigation of the effect of thyroid preparations on dogs. She comes to the conclusion 252