This report describes the development of superoxide dismutase, glutathione peroxidase, and catalase activities in fetal (days 16–22) and neonatal (day 2 postpartum) lungs of normal rats. Each of the enzymes showed an individual pattern of development in the perinatal period. Glutathione peroxidase activity increased by 135% (p< 0.001) during the last 3 days before birth, catalase activity by 105% (p< 0.01) during the first 2 postnatal days, and the activity of superoxide dismutase by 52% (p< 0.05) from gestational day 19 to 2 days after birth. Contamination by superoxide dismutase from blood in the lung samples accounted for less than 2% of the activity. In contrast, not less than 10–30% of glutathione peroxidase.