It has been shown by many workers that a cystic glandular hyperplasia of the uterine mucosa can be induced in various species by prolonged administration of follicular hormones. There may occur, also, a metaplasia of the epithelium of the endometrium, which becomes stratified in certain areas, while the glands may be transformed into solid epithelial cords. Under similar experimental conditions metaplastic changes may occur also in the uterine cervix (for literature see Gardner, 1, and Loeb, 2). These observations are of special importance since the metaplastic changes, which sometimes assume a precancerous aspect, may persist even when administration of estrogens is suspended. In one case atypical growth of the uterine epithelium of precancerous aspect was observed in a castrated guinea-pig which had received subcutaneous injections of the monobenzoic ester of estradiol for a period of eight months, though the injections were suspended four months before the animal was sacrificed (Lipschutz, Iglesias and Vargas, 3). Allen and his associates (4) were successful in transplanting a cancerous cervical tumor occurring in a mouse treated with estrogens for ten months. It may be objected that in all these experiments an artificial condition is created and that no conclusions can be based upon them as to spontaneous tumorigenesis by endogenous estrogens, i.e. by estrogens secreted by the ovary in situ. On the other hand, it was shown by the senior writer some years ago that a similar atypical epithelial growth of the uterine mucosa may be induced in the guinea-pig without the injection of estrogens. If one ovary is removed completely and the second partially, so that only a small ovarian fragment remains in the body (“complete ovarian fragmentation”), a considerable change in the sexual cycle occurs: the follicular phase is prolonged and the vaginal mucosa may remain cornified for many weeks (Lipschiitz, 5). Under these conditions a cystic glandular hyperplasia of the endometrium may occur in the guinea-pig.2 When such animals are kept under observation for several years there is found in almost all of those in which the ovarian fragment has remained intact, a penetration of uterine glands between the muscular coats of the myometrium. There may also be a cornification of the upper part of the cervix, where normally only mucified epithelium occurs. In one instance the aspect of the cervix was frankly precancerous (Lipschutz, 7).