Prolonged anoxia in turtles is associated with marked disturbances in plasma composition. This study examines brain and cerebrospinal fluid (CSF) ion homeostasis in the freshwater turtle, Chrysemys picta bellii, in response to 8-10 days of submergence anoxia at 10 degrees C. For comparison, it also examines the response to experimental elevation of plasma [K], [Ca], and [Mg] in normoxic turtles. Long-term anoxia resulted in marked changes in brain and CSF composition. These included elevated [K], [Ca], [Mg], and [lactate] and reduced [Cl], and the composition of CSF approached that of plasma. Brain water content increased by 17%, which we suggest was an intracellular edema linked largely to an 11% increase in total brain K. In contrast to the lack of effective homeostasis during anoxia, CSF composition was controlled in normoxic animals. We conclude that there is homeostasis of K, Ca, and Mg in the extracellular fluids of normoxic turtle brain, as in other vertebrates, but that this homeostasis fails during long-term anoxia.