Electrophysiological studies were performed on slowly adapting cells of the crayfish (Astacus astacus) stretch receptor to examine some aspects of the operation of the sodium pump. Intracellular sodium activity (aiNa) and pH (pHi) were measured with liquid ion exchanger microelectrodes and the effects of NH3/NH+4 were observed. In cells in which the sodium pump was inhibited by K+-free solution, NH+4 induced a decrease of aiNa that can be explained only in Na+ extrusion is assumed. pHi measurements provide indirect evidence that NH+4 was taken up at the same time as Na+ was extruded. Ouabain blocks the operation of the sodium pump in the presence of K+ and NH+4. This result suggests that the ammonium-mediated decrease in aiNa in K+-free solution was caused by activation of the sodium pump. The results obtained by electrophysiological methods in a living cell are qualitatively in good agreement when compared with biochemical investigations on assays of crustacean Na+-K+ ATPase.