The effects of epidermal growth factor (EGF) on ornithine decarboxylase (ODC) activity and DNA synthesis were studied during the perinatal period in rats. EGF administration to neonatal rats increased ODC activity and DNA synthesis in neonatal rat liver, but not in brain. EGF administration to maternal rats increased ODC activity in maternal rat liver, but not in rat placenta and fetal rat liver. These data suggest that EGF has mitogenic effects on neonatal and maternal rat liver and play some important roles in fetal and neonatal growth.