Female mice of the C57 Black/Tw strain given 5 daily injections with 100 μg testosterone (T) or 5α-dihydrotestosterone (DHT) from the day of birth showed estrogenindependent persistent proliferation and cornification of the vaginal epithelium in adulthood. The vaginal epithelium of the mice was essentially similar to that of the controls in histological structure during or shortly after neonatal injections of the androgens. In T-and DHT-mice aged over 20 days, however, a marked proliferation with or without superficial cornification took place in the epithelium lining the proximal and middle parts of the vagina (Müllerian vagina), while neither proliferation nor cornification occurred in the epithelium of the distal vagina (urogenital sinus vagina). On the second day of postnatal life in mice given a single injection with T on the day of birth, the mitotic activity in the epithelium of the middle vagina was heightened, but it dropped to the control level on the third day and remained low until 20 days. By contrast, the mitotic rates in the epithelium of the rest of the vagina in T-mice and of all parts of the vagina in DHT-mice were approximately the same as in the controls until 20 or 30 days. The mitotic rates in the epithelium of the Müllerian vagina were markedly elevated in T-mice at 20 days of age and DHT-mice at 30 days, and thereafter remained almost unchanged until 60 days of age. These results were different from the findings in mice given neonatal injections with the dose of estradiol-17β(E) capable of estrogen-independent vaginal cornification (Iguchi et al., 1976). The present findings seem to indicate that the mechanism involved in the induction of estrogen-independent vaginal changes by neonatal administration of androgen (T, DHT) is different from that following neonatal treatment with estrogen (E), although androgen and estrogen act directly on the vaginal epithelium of neonates.