Manganese superoxide dismutase (MnSOD) is one of the main antioxidant enzymes in mammalian tissue. Previous studies have shown that the activity of MnSOD increases in the rat kidney during development. To define further the developmental change in MnSOD activity and better understand some of the steps involved in the control of MnSOD expression during kidney development, we measured MnSOD messenger RNA and enzyme activity in the ovine kidney cortex during fetal life, in the newborn period, and in adults. MnSOD mRNA and enzyme activity were detected at 0.65 gestation. Hybridization of the Northern blot with a human MnSOD cDNA probe showed evidence of two transcripts of 4.0 and 1.5 kb, respectively. There was a significant increase with age of MnSOD activity and MnSOD mRNA (p < 0.0001). The abundance of each MnSOD transcript significantly increased with age (p < 0.001). In the fetuses, both transcripts increase in parallel; in newborns and adults the 1.5-kb increase was significantly greater than the 4.0-kb increase. Enzyme activity and mRNA were strongly correlated (r = 0.89, p < 0.0001). These data indicate that the expression of MnSOD is developmentally regulated in the ovine kidney cortex. This increase seems to be dependent largely on pretranslational events.