The intracellular Na activity (aiNa) of sheep Purkinje fibres bathed in solutions with and without K (Rb) is studied by means of Na sensitive microelectrodes. Perfusion with K (Pb) free media increases aiNa. Upon reapplication of K (Pb) containing solutions aiNa decreases and cell membrane hyperpolarizes transiently. After an initial delay attributed to K (Rb) equilibration in the extracellular space, the decline of aiNa and membrane potential towards their respective resting values is approximately monoexponential and displays the same time constant (tau). The tau values and aiNa in the steady state vary with the extracellular K (Rb) concentration ([K]0, [Rb]0). According to a simple model the activation of the Na pump by external K (Rb) can be estimated from the time constants or the aiNa steady state values at various [K]0 ([Rb]0). Half maximal activation occurs at 1.6-3.7 mM K (Rb). External K and Rb ions are equipotent activators of the Na pump in sheep Purkinje fibres.