The mechanism by which betamethasone induces Na-K-ATPase activity in developing tissue was studied in homogenates of proximal tubular cells from 10-day-old rats. A significant increase in Na-K-ATPase activity occurred after 5 micrograms . 100 g-1 . 12 h-1 X 2 beta-methasone and a maximal increase after 15-60 micrograms . 100 g-1 . 12 h-1 X 2. Following a single dose of 60 micrograms . 100 g-1 betamethasone Na-K-ATPase activity increased significantly after 16 h and maximally after 24-30 h. The 16-h time lag suggests that betamethasone does not act only directly on Na-K-ATPase synthesis. Betamethasone 60 micrograms . 100 g-1 increases Na-K-ATPase activity significantly in kidneys in which glomerular filtration rate is reduced by ureteral ligation, but the increase is significantly less pronounced than in kidneys with intact ureters, suggesting that the induction is not mediated only by alterations in sodium supply. Twenty-four hours after 10-60 micrograms . 100 g-1 betamethasone there was no significant increase in glucose-6-phosphatase and Mg-ATPase activity in 10-day-old rats or in Na-K-ATPase activity in 40-day-old rats. The basal and lateral cell membranes of the proximal tubular cells were not significantly increased 24 h after 60 micrograms . 100 g-1 betamethasone. Accordingly, structural development is not a prerequisite for enzymatic differentiation.