Postnatal injections of a single dose of testosterone propionate or estradiol benzoate to newborn female hamsters produce a precocious vaginal opening at 5 days of age. Vaginal postnatal development, histologically, has shown that in testosterone-propionate-injected animals sinusal vagina is absent. The Müllerian vagina presented a stimulated epithelium at early ages. After 40 days, the Müllerian epithelium was stratified keratinized, with zones of stratified cuboidal epithelium. In estradiol-benzoate-treated animals, the Müllerian epithelium rapidly changes to stratified keratinized epithelium (10 days), with zones of stratified cuboidal after 20 days of life. The sinusal epithelium was stratified squamous at 10 days, and after 40 days changes to stratified keratinized epithelium were seen. Probably, early changes are due to a direct effect of steroid action. Later effects are due to ovarian secretion. Histologically, no significant changes were observed in the preputial and urethral glands. The clitoris showed malformatio