SummaryNormal and irradiated female mice were given gonadotropic antiserum. Controls received normal rabbit serum, sheep heart antiserum, or normal saline solution. After antihormone serum treatment ovarian and uterine weights were below those of controls. Ovaries of mice after injection of anti-hormone serum had fewer corpora lutea than did controls, and interstitial tissue gave evidence of diminished gonadotropic stimulation as indicated by increased frequency of cells indicative of gonadotropic deficiency. Uteri lacking histological evidence of stimulation were found most frequently in animals given antihormone serum. Ovaries and uteri of irradiated animals responded in essentially similar manner to treatment, but the response was more extreme. Irradiation and gonadotropic antiserum increased gonadotropic potency of pituitary glands; neither normal rabbit serum nor sheep heart antiserum had any effect. A pattern of vaginal cycling was observed after antihormone treatment which was not apparent in cycle...