Acute and late effects of neonatal estrogen treatment were studied in NMRI mice treated with diethylstilbestrol (DES) or estradiol-17β (E 2) on days 1 to 5 after birth (estrogenized females). The uterine wet weight (UWW) response in 6-day-old females, after 5 daily treatments with DES, had a peak at a daily dose of 10 −2 μg DES and declined with higher doses. Females (26-day-old) treated with DES or E 2 neonatally had a reduced UWW response to a challenge with DES; on a dose basis, DES was more effective neonatally than E 2. A single injection with DES or E 2 in the neonatal period stimulated mitotic activity in the uterine horn epithelium; the UWW response to a 24-h DES pulse increased from day 2 to 6 after birth, but the uterine epithelial mitotic rate response decreased. Epidermal growth factor (EGF) was a more potent stimulator of mitotic activity than DES or E 2. DES inhibited mitotic activity in the uterine cervical epithelium; EGF protected from this DES effect. In adult estrogenized females; EGF-induced uterine stimulation of 3H-thymidine incorporation subsided more rapidly than in control females; uterine epithelium did not respond to EGF in vitro. Uterine stroma of adult estrogenized females is postulated to house a population of cells under nonovarian proliferation control while the uterine epithelium may be under influence of an ovary-dependent proliferation inhibiting factor that is gradually lost under culture conditions.