We have studied the pro-antioxidant status of the rat liver on the last day of gestation and at 1, 15, and 30 days of extrauterine life. Representative variables, such as activities of superoxide dismutase (SOD), catalase, and glutathione peroxidase and concentrations of reduced glutathione and 8-hydroxy-2'-deoxyguanosine, were determined in liver to assess the degree of birth-associated oxidative stress during the fetal-neonatal transition and early development of the rat. Percentages by which liver Cu/ZnSOD activity increased over the basal value of the fetal liver were 54%, 95%, and 127% at neonatal days 1, 15, and 30, respectively. There was a lack of induction in the development profile of MnSOD. Catalase activity was clearly and progressively induced with time from the fetal state up to the neonatal age of 1 month. Glutathione peroxidase activity and glutathione content showed a tendency to decline during the first day after birth, though they increased to significantly higher values on days 15 and 30. However, the amount of rat liver 8-hydroxy-2'-deoxyguanosine did not increase. These results suggest that the induced antioxidant activities may be responsible for maintaining DNA stability during the perinatal development of the rat liver.