Neonatal, adult, and fetal rat lungs of 18, 20, and 22 d gestation from four to six litters were examined for cytochrome oxidase, glucose-6-phosphate dehydrogenase, catalase, glutathione peroxidase, copper-zinc and manganese superoxide dismutase activities. All results were corrected for the contribution of enzymes in blood that contaminate homogenates. Because lung protein/DNA ratios and body water change significantly with gestational age, enzyme activities were expressed as U/mg DNA. All activities were low in d 18 lung and increased with advancing gestational age. Only catalase and copper-zinc superoxide dismutase increased activity in response to air breathing, suggesting that maturation of the antioxidant enzyme system is virtually complete before delivery. Activities of glucose-6-phosphate dehydrogenase, catalase, glutathione peroxidase, and manganese superoxide dismutase were higher in neonatal than in adult lung.