Fifty years have passed since Overton reported in 1902 on the significance of extracellular Na+ for the excitability and contractility of muscle, and found the debilitating effect of increased extracellular K+ concentrations. The issues raised by this discovery have since engaged more than one generation: it is the fundamental question of how the Na+ ions in the extracellular space and the K+ ions inside the cell participate in the muscle contraction and in the conduction of excitation. As significant as the experimental facts are, the considerations that Overton connected to his findings in 1902 seem equally important today: even back then, Overton suggested the possibility that during muscle excitation, intracellular K+ might need to exchange with extracellular Na+. The relationship of this ion process is also noteworthy.