With the aid of sodium-sensitive glass electrodes, changes in sodium ion activity were studied in the course of subsequent additions of components required for ATP hydrolysis provided by Na(+)-K(+)-dependent membrane ATPase. Membrane ATPase was obtained from guinea pig kidney cortex. In the presence of ATP, Mg(++) and Na(+) in media, the addition of K(+) caused an increase in Na(+) activity. The omission of ATP or its substitution by ADP as well as the addition of Ca(++) to the media eliminated the above-mentioned increase of Na(+) activity. Quabain did not affect Na(+) release caused by the addition of K(+), although it significantly inhibited ATPase activity of the preparation. The data obtained were considered to be a direct indication of ion exchange during the course of membrane ATPase reaction. This ion-exchange stage of the reaction is not inhibited by ouabain. The ratio of sodium ions released per one inorganic phosphate formed in the course of the reaction was found to be much higher than that established for transporting membranes of intact cells. A possible cause of this difference is discussed.