The extent of lipid peroxidation and the levels of its antioxidants such as superoxide dismutase (SOD), catalase and glutathione peroxidase (GSH-Px) were determined on lung tissues of the fetal, newborn and adult rat. Lipid peroxide formation was slight in the fetal period but augmented after birth reaching a peak at about 10 days after birth. The peroxide concentration then gradually declined with development and the adult level was found comparable to the fetal level. In the examination of the developmental defensive mechanism on the basis of assays for the aforementioned antioxidant enzymes in lung tissue, the SOD activity was low in fetuses reaching approximately 90% of the adult level at 10 days of life. Catalase was extremely low in concentration at all times, and age-related variations could not be definitely obtained. GSH-Px was also measured low in the fetal period and during 20 days after birth, but a subsequent gradual rise resulted in threefold greater activity in adults than in fetuses.