Administration of diethylstilbestrol (DES) to neonatal mice causes a high incidence of vaginal adenosis comparable to adenosis in women who were exposed to DES in utero. In the present study, scanning electron microscopy was used to investigate morphological events that might be involved in the development of adenosis. The effects of DES on luminal epithelia of neonatal mouse uterus, vaginal canal, and vaginal fornix were compared with the effects of the drug on those tissues of the adult mouse. BALB/c mice were given five s.c. injections of 2 micrograms DES in sesame oil or of sesame oil alone (controls) on postnatal Days 1 through 5 and were killed on Day 7. Ovariectomized adult BALB/c mice were given s.c. injections of 2 or 20 micrograms DES or of oil on 5 consecutive days and were killed 2 days after the last injections. In neonatal and adult uteri, DES stimulated the growth of microvilli on epithelial cells. The numbers of long and intermediate-length microvilli were markedly increased relative to those in control uteri. DES caused epithelial cornification in all samples of adult vaginal canal and fornix and in three of eleven Day 7 vaginal canal samples, but not in the remaining eight Day 7 vaginal canal samples or in any of ten Day 7 fornix samples. In the latter tissues, DES caused a marked increase in numbers of short, long, and intermediate-length microvilli on the apical surfaces of epithelial cells. Thus, in the majority of cases, en Day 7 vaginal canal samples, but not in the remaining eight Day 7 vaginal canal samples or in any of ten Day 7 fornix samples. In the latter tissues, DES caused a marked increase in numbers of short, long, and intermediate-length microvilli on the apical surfaces of epithelial cells. Thus, in the majority of cases, en Day 7 vaginal canal samples, but not in the remaining eight Day 7 vaginal canal samples or in any of ten Day 7 fornix samples. In the latter tissues, DES caused a marked increase in numbers of short, long, and intermediate-length microvilli on the apical surfaces of epithelial cells. Thus, in the majority of cases, the response of neonatal fornicovaginal epithelium to DES resembled that of neonatal and adult uterine epithelium more than it resembled the response of adult fornicovaginal epithelium; the uterine-like response might be causally related to the development of adenosis.