Electron probe x-ray microanalysis (EPMA) was used to measure water content (percent water) and dry weight elemental concentrations (in millimoles per kilogram) of Na, K, Cl, and Ca in axoplasm and mitochondria of rat optic and tibial nerve myelinated axons. Myelin and cytoplasm of glial cells were also analyzed. Each anatomical compartment exhibited characteristic water contents and distributions of dry weight elements, which were used to calculate respective ionized concentrations. Free axoplasmic [K+] ranged from approximately 155 mM in large PNS and CNS axons to approximately 120-130 mM in smaller fibers. Free [Na+] was approximately 15-17 mM in larger fibers compared with 20-25 mM in smaller axons, whereas free [Cl-] was found to be 30-55 mM in all axons. Because intracellular Ca is largely bound, ionized concentrations were not estimated. However, calculations of total (free plus bound) aqueous concentrations of this element showed that axoplasm of large CNS and PNS axons contained approximately 0.7 mM Ca, whereas small fibers contained 0.1-0.2 mM. Calculated ionic equilibrium potentials were as follows (in mV): in large CNS and PNS axons, E(K) = -105, E(Na) = 60, and E(Cl) = -28; in Schwann cells, E(K) = -107, E(Na) = 33, and E(Cl) = -33; and in CNS glia, E(K) = -99, E(Na) = 36, and E(Cl) = -44. Calculated resting membrane potentials were as follows (in mV, including the contribution of the Na+,K+-ATPase): large axons, about -80; small axons, about -72 to -78; and CNS glia, -91. E(Cl) is more positive than resting membrane potential in PNS and CNS axons and glia, indicating active accumulation. Direct EPMA measurement of elemental concentrations and subsequent calculation of ionized fractions in axons and glia offer fundamental neurophysiological information that has been previously unattainable.