The effects of aging on active Na+ extrusion and oxygen consumption associated with it were studied in rat kidney cortex cells. It was found that (a) the active extrusion of Na+ undergoing Na/K exchange and the active extrusion of Na+ with Cl–– and water were diminished in old rats (24 months) as compared with young rats (3 months); (b) the oxygen consumption associated with each of the two active mechanisms of Na+ extrusion was also diminished in the old rats; (c) the calculated turnover rate of the Na/K pump was significantly lower for the old r