The distribution of the estrogen receptor (ER) was investigated immunohistochemically in the genital tract of 0 (day of birth)--to 50-day-old C57BL/Tw female mice given 5 daily injections of 2 micrograms diethylstilbestrol (DES) from the day of birth using an anti-ER monoclonal antibody. In the oviduct, cervix and vagina of control mice, both epithelial and stromal cell nuclei showed positive ER immunoreaction at 0 day. Nuclei of uterine stromal cells also showed strong ER immunostaining even at 0 day. In contrast, the ER in uterine epithelial cells began to appear by day 5. The number of positive epithelial cells and staining intensity gradually increased until 10 days. In neonatally DES-exposed mice, ER appeared in uterine epithelial cells 24 h after the injection. Polygonal cells appearing in the vaginal epithelium of DES-exposed 3-day-old mice showed weak ER staining. Neonatal DES exposure reduced staining intensity of columnar vaginal epithelial cells and uterine stromal cells at 3 days. Vaginal epithelial cells undergoing ovary-independent stratification and cornification in neonatally DES-exposed mice and normal estrous mice showed ER immunoreaction only in the basal cells.