The growth processes induced by estrogenic hormones which take place in the uterus of the mouse are investigated. The uterus and other sex organs were studied in 501 mice including 177 control mice which were not subjected to sex hormones. 3 different strains of mice of ages ranging from 1-20 months were used. Mice were variously injected with 1-100 rat units of estrogen, estrogen and acid extract of cattle anterior pituitary gland, acid extract alone, estrogen and corpus luteum extract (Proluton), and corpus luteum alone. Changes which may take place in the uterus of mice under the influence of estrogen include: 1) the normal one-layered cylindrical epithelium may change into several rows of cylindrical cells or into a partly cylindrical and partly squamous epithelium, or a typical squamous epithelium; 2) gland ducts may become cystically dilated and filled with colloid material; 3) the uterine glands may proliferate and penetrate into and through the muscle tissue; 4) infection may take place; and 5) parts of the pancreas may become adherent to the uterus. In none of the test cases were cancerous changes or even precancerous proliferations found. Reactivity of the uterine epithelial structures to growth stimulation is less than that of the compounding tissues in the vagina, cervix, and mammary gland. Cancerous transformation depends, among other factors, on the product of the intensity of growth stimuli acting on a tissue and the responsiveness of the affected tissue. In a minority of the mice certain changes were observed such as a penetration of the uterine glands into or through the musculature of the uterus and a metaplasia of the cylindrical surface epithelium. In a considerable number of cases, the squamous epithelium owed its origin to regenerative processes which led to an extension of the cervix epithelium into the uterus. Under the influence of estrogen these changes are produced more readily the greater the dosage used and the more continuous the action.