It has been reported that levels of antioxidant enzymes are low in fetal rat lungs and kidneys, and that they increase rapidly during late gestation. Among the antioxidant enzymes, both copper-zinc and manganese superoxide dismutases (CuZnSOD and MnSOD) are assumed to play a key role in protection against oxidative tissue injury. To determine the nature of the rapid perinatal increase in CuZnSOD and MnSOD, immunoenzyme staining was performed in the lungs and kidneys of fetal (d 18 and 20 of gestation) and neonatal (d 22) rats. The CuZnSOD and MnSOD in the homogenates were assayed by RIA, and they were found to be higher in the neonatal organs than in the respective fetal organs. The neonatal bronchiolar epithelium was stained for both CuZnSOD and MnSOD more intensely than the fetal one. The CuZnSOD staining in the neonatal alveolar wall was more intense than that in the fetal one. There was a significant reactivity for MnSOD in the neonatal, but not in the fetal, alveolar walls. In the kidneys, the reactivities for CuZnSOD and MnSOD were confined to the undifferentiated tubules. Although the tubules were increased in numbers in the neonatal kidneys, the intensity of the staining for both CuZnSOD and MnSOD was unchanged. The histochemical study disclosed that CuZnSOD and MnSOD increased in the kidneys in a manner different from that in the lungs. The low concentration of both CuZnSOD and MnSOD in the fetal lung tissues may contribute to the vulnerability to oxygen toxicity. Such changes in the concentrations in specific tissues were not delineated in the kidneys.