Abstract— The activities of (Na+ K+)-ATPase and its proposed partial reactions, K +-pNPPase and Na +-dependent phosphorylation, all increase tenfold relative to microsomal protein between 5 days prior to birth and 60 days postnatally in NaI-treated rat cerebral microsomes, and all reach half of their adult values between the fifth and tenth postnatal day. These increases are concurrent with the most rapid changes in cerebral wet weight. Increases in the amount of the related phosphorylatable polypeptide during development. as estimated by densitometry of Coomassie-stained polyacrylamide gels after electrophoresis of constant amounts of microsomal protein dissolved in sodium dodecylsulfate, parallel the increments in levels of Na +-dependent phosphorylation. The fraction of total phosphorylation that is Na +-dependent increases steadily during development. suggesting a precursor role for some of the Na +-independent fraction. The results are consistent with a single biosynthetic control for the enzymatic sites critical to the partial reactions of (Na +-K +)-ATPase. No changes in turnover number or affinity for substrate or ligands were found during development. Little similarity was noted among the age-related changes of Mg 2+ -ATPase activity. Mg 2+ -paranitrophenylphosphatase activity, and Na+-independent phosphorylation levels. The most rapid changes in (Na+-K+)-ATPase take place during the period corresponding to glial proliferation and neuronal arborization.