Electron probe x-ray microanalysis was used to measure water content and concentrations (mmol/kg dry weight) of elements (Na, P, S, Cl, K, Ca, and Mg) in myelinated axons and glial cells of rat optic nerve exposed to in vitro anoxia and reoxygenation. In response to anoxia, large, medium, and small diameter fibers exhibited an early (5 min) and progressive loss of Na and K regulation which culminated (60 min) in severe depletion of respective transmembrane gradients. As axoplasmic Na levels increased during anoxic exposure, a parallel rise in Ca content was noted. For all axons, mean water content decreased progressively during the initial 10 min of anoxia and then returned toward normal values as anoxia continued. Analyses of mitochondrial areas revealed a similar pattern of elemental disruption except that Ca concentrations rose more rapidly during anoxia. Following 60 min of postanoxia reoxygenation, the majority of larger fibers displayed little evidence of recovery, whereas a subpopulation of small axons exhibited a trend toward restoration of normal elemental composition. Glial cells and myelin were only modestly affected by anoxia and subsequent reoxygenation. Thus, anoxic injury of CNS axons is associated with characteristic changes in axoplasmic distributions of Na, K, and Ca. The magnitude and temporal patterns of elemental Na and Ca disruption are consistent with reversal of Na(+)-Ca2+ exchange and subsequent Ca entry (Stys et al., 1992). During reoxygenation, elemental deregulation continues for most CNS fibers, although a subpopulation of small axons appears to be capable of recovery.