This study describes the gradual changes which take place in the epithelium of vagina and cervix of mice from early to advanced age, with and without the stimulation of estrogen and other hormones, and the eventual transition from normal growth processes to precancerous and cancerous proliferation. With advancing age the average depth of the epithelial processes extending from the surface epithelium into the underlying connective tissue in the vagina and cervix of mice increases. Estrogenic substances injected into these animals causes these processes to reach a greater depth (10.7%) than in noninjected controls (0.8%). Injection of estrogenic substances over long periods of time causes these processes to pass into a precancerous and into a carcinoma-like growth. Mechanical factors may lead to partial erosion of the surface epithelium. Under the influence of intense stimulation leading in the end to carcinoma-like growth processes in the vagina and cervix, morphological potentalities of the tissues may be revealed which under ordinary conditions would remain hidden. These observations confirm that the transitions in the epithelial tissue of vagina, cervix, and uterus are gradual. Further research is needed to investigate the occurrence of spontaneous carcinoma, which is a condition analagous to that seen in the mammary gland.