IF THE inbred mouse of Strong’s C3H strain is gonadectoraized the adrenal cortex becomes hyperplastic and in the female it produces enough sex hormones to stimulate the uterus and vagina (Woolley and Little, 1945). The morphological alteration of the adrenal is evident as early as thirty-seven days after ovariectomy, and extends to a large part of the cortex in three to six months (Casas, King and Visscher, 1949). If the same strain is given thiourea, the adrenal cortex becomes atrophic (Dalton, Morris and Dubnik, 1945). The study reported here was undertaken to ascertain the effect on the adrenal cortex of subjecting the C3H mouse to two apparently antagonistic influences, namely, thiourea to induce atrophy and castration to enhance hypertrophy. MATERIALS AND METHODS The animals used in this experiment were inbred C3H mice from the colony raised at the School of Medicine, School of Tropical Medicine. These were descendants of mice given to the institution by Dr. J. J. Bittner, of the Department of Cancer Biology of the University of Minnesota.