An immunocytochemical assay for estrogen receptor (ER) was used to study the distribution of receptor in fetal and immature female mouse reproductive tracts. Immunoblots confirmed that a single band, the size of the ER, immunostained in extracts from day 15 and 17 fetal reproductive tracts. Staining was observed over nuclei of epithelial cells of the Mullerian duct and over nuclei of cells of the developing connective tissue (mesenchymal cells) of the reproductive tract on fetal day 15. By day 17 when a primitive uterus could be distinguished, ERs were detected in nuclei of mesenchymal cells, but in only a small portion of epithelial cells. A different pattern of immunocytochemical staining was observed in uteri from animals killed on the day of birth; cells of the connective tissue contained ER, but the epithelial cells did not. By 4 or 6 days after birth, more nuclei in the connective tissue stained for ER with a greater intensity compared to nuclear staining in epithelial cells. ERs were detectable in nuclei of both uterine epithelial cells and connective tissue cells on days 10 and 19 after birth.