The control of ion fluxes to and from bone has important implications to mineralization and calcium homeostasis. Since ionic transport frequently results in an electrical potential difference across the layer of cells lining bone surfaces, knowledge of this potential is critical to understanding the means of regulation of ionic concentrations in the interior bone fluid phase. This work presents a determination of a metabolism-related electrical potential difference by a thermodynamic argument based on the distribution of charged and uncharged tracers between the bone extracellular fluid compartment and the bathing medium. For embryonic chick calvaria whose viability was assured by oxygen consumption measurements, an electrical potential of 4 mV was determined, positive with respect to the bone fluid compartment. This measurement is shown to be consistent with the active transport of K+ ion into the interior of bone; the potential developed by this transport process will then exclude from the interior phase a passively leaked Ca2+ ion.