An attempt was made to determine whether or not the steroid hormones, aldosterone and betamethasone, increased the Na‐K‐ATPase activity and the surface density of baso‐lateral cell membranes in the juxtamedullary proximal convoluted tubules of young and adult mice. The results showed that aldosterone and betamethasone increase the enzyme activity in young mice but not in adult mice. However, the surface density of the baso‐lateral cell membranes did not increase either in the young or in the adult mice. Further, the induction ratio for the Na‐K‐ATPase activity did not differ between the two hormone‐treated groups in young mice. The results suggest that a difference in hormone sensitivity exists between the proximal convoluted tubular cells of young mice and those