To provide evidence of active accumulation of K⁺ in bone extracellular fluid (BECF), electric currents driven by damaged living metatarsal bones of weanling mice, immersed in physiological media at different [K⁺], in the presence of blockers of the K⁺ channels or of the Na⁺-K^+-ATPase inhibitor, were measured by means of a voltage-sensitive two-dimensional vibrating probe. At 4 mM extracellular K⁺ concentration ([K⁺]_o), an inward steady current density (7.85-38.53 µA/cm²) was recorded at the damage site, which was significantly dependent on [K⁺]_o. At [K⁺]_o equal to that of BECF (25 mM), current density was reduced by 76%. At [K⁺]_o of 0 mM, the current density showed an increase, which was hindered by tetraethylammonium (TEA). Basal current density was reduced significantly after exposure to TEA or BaCl₂ and was unchanged after long- term exposure to ouabain. By changing control medium with a chloride-free medium, current density was reversed. The results support the view that K⁺ excess in bone is maintained by a biologically active cellular system. Because the osteocyte-bone lining cell syncytium was at the origin of the current in bone, it is likely that this system controls the ionic composition of BECF.