The Na,K-ATPase is a critical protein for cellular homeostasis. It transport three sodium ions for two potassium ions across the plasma membrane and utilizes the energy from ATP hydrolysis to transport these cations against the chemoelectric gradient. By combining electrophysiology and fluorescence-based methods, we have previously been able to measure the conformational dynamics of the holoenzyme and to determine distance constraints between subunits of the enzyme (1, 2). In our current experiments, voltage clamp fluorometry was used to monitor conformational changes associated with electrogenic partial reactions of the Na,K-ATPase after changes in the concentration of internal sodium or external potassium (3). To probe the effects of the internal sodium concentration on the sodium branch of the Na,K-ATPase, Xenopus laevis oocytes were depleted of sodium and then loaded with sodium using the amiloride-sensitive epithelial sodium channel. The potassium branch of the Na,K-ATPase was studied by exposing the oocytes to different potassium concentrations in the presence and absence of internal sodium to obtain information on the apparent affinity for external potassium. Our results provide evidence on the relationship between lowering the internal concentration of sodium and increasing the amount of external potassium. These experiments demonstrate that it is possible to examine how external and internal ligands affect the conformational equilibrium of the ion pump on the surface of whole cells.