NUMEROUS PAPERS have been published in the past few years showing that thiourea suppresses the formation of thyroxin by the thyroid gland, although the exact mechanism has not been fully explained. Since thyroxin is such an important metabolic stimulant, the experiments here reported were started to determine whether this suppression of thyroxin formation affected the neutral fat and cholesterol content of the animal's bodies, skins, and livers. Studies are also reported on the possible inhibiting action of the feeding of dried thyroid gland on the changes from the thiourea administration. In one experiment liver glycogen was also determined since the histological picture of the liver indicated a high glycogen content of the livers of the thiourea fed animals. Young Albino rats of the Wister strain were used in all the experiments. In experiment 1 the animals were fed Rockland rat diet without additional vitamin supplements. Diets of known composition were used in experiments 2-10.