Estradiol (E2) stimulates epithelial growth in the female genital tract via estrogen receptors (ER) in the stroma using paracrine mechanisms. Keratinocyte growth factor (KGF), a member of the fibroblast growth factor family, is produced by mesenchymal cells and is mitogenic for epithelial cells making it a strong candidate as a paracrine mediator. Transcripts for KGF and the KGF receptor were detected in the neonatal mouse uterus and vagina. Treatment of neonatal mice with KGF elicited changes in uterine and vaginal epithelium within five days and induced long term effects in these tissues. Newborn female Balb/c mice were injected daily with 5 microg/g body weight of KGF or saline for five days. KGF-treated mice exhibited a 5- to 6-fold increase in uterine epithelial BrdU-labeling index and a 4- and 5-fold increase in vaginal epithelial BrdU-labeling index vs. respective saline-treated controls. Histological sections of KGF-treated uteri revealed dramatic increases in epithelial surface area due to extensive folding of the luminal epithelium. In some areas, the evaginated luminal epithelium invaded zones normally occupied by myometrium. Vaginal epithelium was thicker than that of saline-treated controls following 5 days of KGF treatment. When KGF-treated newborn mice grew to adulthood and were ovariectomized, vaginal smears exhibited persistent diestrus in all animals. Histologic analysis demonstrated a thick parakeratotic vaginal epithelium (approximately 10 cell layers) 9 days postovariectomy in adult neonatally KGF-treated mice. Our studies indicate that KGF injected into neonates markedly stimulated proliferation of neonatal uterine and vaginal epithelium and elicited long-term, persistent abnormal changes in vaginal epithelium.