Autoradiographic analysis of [3H]-estrogen nuclear binding sites was performed on developing genital tracts (uterus, cervix and vagina) of mice 1 to 90 days postpartum. During days 1 to 15 postpartum, nuclear estrogen binding sites were observed exclusively within mesenchymal cells; epithelial cells did not exhibit nuclear labelling. At 18 days postpartum vaginal and cervical epithelial cells exhibited nuclear estrogen binding sites for the first time, whereas the initial appearance of estrogen receptor activity in the epithelium of the uterus was detected at 20 days postpartum. Thereafter, nuclear estrogen binding sites were maintained in both epithelial and stromal cells into adulthood. The acquisition of nuclear binding sites within epithelium of female genital organs at 18 days is discussed in terms of epithelial-mesenchymal interactions and the acquisition of growth responsiveness.