We examined the chronology of development of both fetal lung antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase) and disaturated phosphatidylcholine ("surfactant") during late gestation in four laboratory animal species: rat, rabbit, hamster, and guinea pig. An essentially similar pattern of prenatal biochemical maturation was found in all four species. The developmental changes were characterized by (1) rapid elevations in fetal lung antioxidant enzyme levels during the final 10% to 15% of gestation, and (2) an essentially parallel rapid rise in lung surfactant content during the final 10% to 15% of gestation. The increase in the lung activity of the individual antioxidant enzymes prior to birth averaged approximately 150% to 200%. Our findings suggest that late gestational changes in the principal pulmonary antioxidant defense system (like the changes in the surfactant system) represents a normal "preparation for birth," required to assure successful functioning of the neonatal lung in the relatively oxygen-rich ex utero environment.