The charge transporting activity of the sodium pump depends on its surrounding electric field. To isolate which steps of the enzyme’s reaction cycle involve charge movement we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with benzyltriethylammonium (BTEA), which binds from the extracellular medium to the sodium pump’s transport sites in competition with Na+ and K+, but isn’t occluded within the protein. We find that only the occludable ions Na+, K+, Rb+ and Cs+ cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This is supported by theoretical simulations based on recently published crystal structures of the sodium pump showing that the bound ions are electrically shielded from the surrounding lipid by the intervening protein. This result appears at first to conflict with electrophysiological studies suggesting that extracellular Na+ or K+ binding in a high field access channel is a major electrogenic reaction of the sodium pump. However, all the results can be explained in a consistent fashion if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.