Studies on transmembrane ion movements in brain cells are of fundamental importance to understanding brain function. Thus, studies on the content and fluxes of major ions such as K+ , Na+ , Cl− , and Ca2+ in neurons are critical to understanding the effects of conductance changes during excitatory or inhibitory events (Hille, 1984; Katz, 1966), and changes in Ca2+ conductance are critical for transmitter release (Hille, 1984; Douglas, 1978) Changes in pHalso affect membrane conductances (Moody, 1983). The complexities of the ion transport processes present in the major nonneuronal cells of the brain (glia, endothelia, and ependyma) are also now beginning to be appreciated and studied, and many of these processes appear to be electrically silent. Such processes appear likely to be involved in control of extracellular ion concentrations and pH and thus will also be important for neuronal function (Varon and Somjen, 1979; Kimelberg and Bourke, 1982; Kimelberg and Ransom, 1986). Exaggeration of such processes may underly the swelling of astroglia frequently seen in various pathological states (Kimelberg and Ransom, 1986). The maintenance of low intracellular sodium concentrations ([Na+ ],) is also important for maintaining inward Na+ gradients for secondary active cotransport of transmitters and other substances into both neurons and glia (Fonnum et al., 1980).