We examined the effects of development, exogenous, and endogenous glucocorticoids on Na+,K+-ATPase activity and subunit protein expression in ovine cerebral cortices and renal cortices. Ewes at 60%, 80%, and 90% gestation, newborns, and adults received 4 dexamethasone or placebo injections. Cerebral cortex Na+,K+-ATPase activity was higher (P <.05) in placebo-treated newborns than fetuses of placebo-treated ewes and adults, α1-expression was higher at 90% gestation than the other ages; α2-expression was higher in newborns than fetuses; α3-expression was higher in newborns than 60% gestation; α1-expression was higher in newborns than the other ages, and α2-expression higher at 60% than 80% and 90% gestation, and in adults. Renal cortex Na+,K+-ATPase activity was higher in placebo-treated adults and newborns than fetuses. Cerebral cortex Na+,K+-ATPase activity was higher in dexamethasone- than placebo-treated adults, and α1-expression higher in fetuses of dexamethasone- than placebo-treated ewes at 60% and 80% gestation. Renal cortex Na+,K+-ATPase activity and α1-expression were higher in fetuses of dexamethasone- than placebo-treated ewes at each gestational age, and α1-expression was higher in fetuses of dexamethasone- than placebo-treated ewes at 90% gestation and in dexamethasone- than placebo-treated adults. Cerebral cortex Na+,K+-ATPase activity, α1-expression, α1-expression, and renal cortex α1-expression correlated directly with increases in fetal cortisol. In conclusion, Na +,K+-ATPase activity and subunit expression exhibit specific developmental patterns in brain and kidney; exogenous glucocorticoids regulate activity and subunit expression in brain and kidney at some ages; endogenous increases in fetal cortisol regulate cerebral Na+,K +-ATPase, but exogenous glucocorticoids have a greater effect on renal than cerebral Na+,K+-ATPase.