In neonatally estrogenized mice, uterine epithelial cells possessed a few microvilli on the fuzzy-appearing apical surface, regardless of the presence or the absence of estrogen. The cells showed well-developed rough endoplasmic reticulum and a large number of mitochondria, suggesting that the cells were actively functioning. At 13 months of age, the uterine epithelium of neonatally estrogenized mice was sometimes stratified and squamous. Spherical basal cells like those appearing in cancerous vagina in estrogen-treated mice made their appearance. These cells may have the capacity of autonomous proliferation.