Histologic changes in the region of the squamocolumnar junction of the uterine cervix in ovariectomized C3H/HeJ mice were studied at 6-12 hour intervals after the injection of 2.3μg estradiol benzoate. Mitotic activity, especially in the basal layer of stratified squamous epithelium, increased as time elapsed after estrogen administration and produced a concomitant increase in stratification of squamous epithelium; some mitotic activity was present in the uterine columnar epithelium throughout the period of observation. The histologic character of the squamocolumnar junction changed from gradual transition to an abrupt demarcation, as in intact mice during the estrous cycle. Subcolumnar cells in the transition zone of untreated animals formed a unilayered reticulum which after estrogen injection developed into a reticulum of stratified squamous epithelium. Extensions of stratified epithelium which develop during chronic estrogen treatment apparently develop from the stratified reticulum. There was no evidence to indicate any contribution of metaplasia to estrogen-induced epidermization.