The evidence that there is intracellular compartmentalization of potassium is indirect but diverse. Intracellular electrode measurement of potassium activity, 42K radioisotope studies, and more recently 39K nuclear magnetic resonance (NMR) all support such compartmentalization. The use of rubidium to apparently shift potassium between sites with different NMR characteristics (visibility) is strong evidence for such compartmentalization. The evidence that intracellular compartmentalization of potassium is of (patho)physiological significance is also indirect. Postulated roles for regulation of intracellular K+ activity, perhaps by control of compartmentalization, include enzyme activity, protein synthesis, and cell growth. There is also evidence that compartmentalization of potassium may contribute to the maintenance of a stable intracellular environment following potassium loading. The apparent magnetic field dependence of the visibility of K+ by 39K NMR offers the opportunity to explore further the phenomenon of compartmentalization.