The distribution of the estrogen receptor (ER) was investigated in neonatal female genital tracts (uterus, oviduct, cervix, and vagina) from days 1-22 after birth, using immunohistochemistry employing an anti-ER monoclonal antibody. In uteri, the ER in epithelial cells began to be observed by day 4. The number of positive epithelial cells and the staining intensity gradually increased until day 22 of age. On the other hand, uterine stroma cells gave a strong ER immunostaining even on day 1. The staining intensity reached a maximum by days 4-7 and then slightly decreased with age. In the oviduct, cervix, and vagina, epithelial cells showed positive ER immunostaining on day 1, and the intensity increased gradually until day 22. ER immunostaining in stroma cells was almost constant during the development period. The ER in both epithelial and stroma cells from these younger animals showed similar biochemical properties, i.e. an increased affinity for nuclei and resistance to extraction with PBS. Thus, during neonatal development of the female reproductive tract, ER is present not only in stroma cells but also in epithelial cells. This ER protein exhibits properties and characteristics similar to those of adult mice. The presence of ER suggests that some of the estrogen actions of cell proliferation, differentiation, and tissue abnormalities resulting from prenatal and postnatal estrogen administration may be mediated by receptor interactions.