The horns of bicornic uterus in the rat develop from the middle parts of Müllerian ducts. Female rat fetuses for this study were taken on 16-21 days of gestation. On the 16th prenatal day the uterine part of Müllerian duct was short, lined with a columnar epithelium and basal lamina around the duct was incomplete. During further development the duct enlarged, epithelial cells became taller and basement membrane was formed. Until birth the epithelium remained relatively indifferent as far as the periductal mesenchyme. Contrary to the animals with a long gestational period and human, the differentiation of uterine wall including uterine mucosa in the rat takes place after birth.