The effect of epidermal growth factor (EGF) on sexual maturation of female rats was studied. A within-litter experimental design was employed, so that in each litter each female received four daily injections of EGF (E, 500 ng/g body weight s.c.) or vehicle (V), at one of three ages: days 0-3 (E1, V1), days 8-11 (E2, V2), days 16-19 (E3, V3). Body weight, pinna detachment, incisor eruption, eye opening, auditory startle, visual placing, vaginal opening and first cytological oestrus were assessed. Neonatal treatment with EGF (E1) delayed pinna detachment and the appearance of the auditory startle, but accelerated eye opening. Also, E1, but not E2 and E3, resulted in lower body weight at weaning. Treatment E3 advanced sexual maturation, as indicated by vaginal opening and first cytological oestrus, by 5-6 days. E1 and E2 had no such effect. Hence the sensitive period for the effect of EGF on female rat sexual maturation is later than that for effects on other developmental characteristics and body weight. In a second experiment, ovary and uterus weights were found not to differ between E3 and V3 females killed on the day of vaginal opening of the E3 rats, suggesting that the effect of EGF may be specifically on the perineal epithelium and not on sexual maturation generally.