Ouabain-sensitive Na+ and K+ fluxes and ATP content were determined in high potassium sheep erythrocytes at different values of membrane potential and internal pH. Membrane potential was adjusted by suspending erythrocytes in media containing different concentrations of MgCl2 and sucrose. Concomitantly either the external pH was changed sufficiently to maintain a constant internal pH or the external pH was kept constant with a resultant change of internal pH. The erythrocytes were preincubated before the flux experiment started in a medium which produced increased ATP content in order to avoid substrate limitation of the pump. It was found that an increased cellular pH reduced the rates of active transport of Na+ and K+ without significantly altering the ratio of pumped Na+/K+. This reduction was not due to limitation in the supply of ATP although ATP content decreased when internal pH increased. Changes of membrane potential in the range between -10 and +60 mV at constant internal pH did not affect the rates of active transport of Na+ or K+.