Both aldosterone and insulin increase active Na+ transport across the urinary bladder of the toad. Recent data have provided further support to the concept that aldosterone acts primarily to increase Na+ entry from the mucosal medium into the transporting cells, whereas insulin acts to increase active Na+ extrusion into the serosal medium. To examine this concept further, the driving force (E(Na)) of the Na+ pump was measured, by the technique described by Yonath and Civan (48), before and after hormonal administration. Both hormones increased short-circuit current, but only insulin increased E(Na). The validity of the technique was further explored by imposing periods of hypoxia upon a series of experimental hemibladders; as expected, hypoxia reversibly decreased E(Na). The data indicate that insulin stimulates Na+ transport, in part by directly stimulating the Na+ pump. The results are also consistent with the concept that aldosterone stimulates net Na+ movement solely by enhancing Na+ entry into the transporting cells, but are subject to alternative interpretations.