The adm. of estrogenic hormone preps. to 88 normal or spayed $ guinea pigs has resulted in the regular occurrence of cystic glandular hyperplasia of the uterine cornua. In animals treated for several mos. this condition is extended to the fundic endometrium. At the bases and tips of the hyperplastic glands the epithelial cells became metaplastic. This condition of metaplasia is most marked in the cervical regions where downgrowths from the epithelium penetrate the submucosa and show pronounced keratinization and pearl formation. The uteri of 15 animals have shown multiple subperitoneal fibromyomatous growths. All animals bearing these tumors have shown periods of spontaneous vaginal bleeding. These periods have been induced in 5 animals by the withdrawal of treatment.