This study examines the normal postnatal (p.n.) development of the Mongolian gerbil's vagina from birth to the age when consistent cyclic changes of the estrus occur. At birth, the Müllerian vagina has a lumen and is lined by a pseudostratified or 2- to 3-layered epithelium. The cells contain large amounts of glycogen. The sinus vagina forms a solid plate and contains hardly any glycogen. During the first few p.n. days the cell layers separate, and the newly formed lumen becomes continuous with the Müllerian vagina. Only a vaginal closure membrane persists. Between p.n. days 10 and 15 the epithelium increases in height reaching 2-5 cell layers. The amount of glycogen diminishes, but in the luminal cells the first mucous granules develop. Leukocyte diapedesis starts, and immature Langerhans cells become discernible. Between p.n. days 18 and 30 the apical cells become mucified, and some shedding of these cells begins. Between p.n. days 36 and 45 bundles of keratin filaments appear in many of the luminal mucous cells intermingled with the mucous granules. In the middle layers keratin filaments, keratohyalin granules and membrane-coating granules indicate the beginning of the keratinization process. Diapedesis of leukocytes may lead to cell aggregations below top layers. By day 50 the shedding of the mucous cell layers reveals the underlying keratinized cell layers which then become exfoliated. The first steps of the estrous cycle show a heterogeneous pattern. At about p.n. day 60 the cycle becomes established.