Brain slices from cerebral cortex represent the classical model with which the first studies of cellular mechanisms of water and ion homeostasis were conducted. These experiments involved the analysis of ion and water movements with radiotracers. The amount of information gained with this approach has been summarized in reviews by Marchbanks (1970), Katzman and Pappius (1973), and Hertz and Schousboe (1975). The difficulty of this method is, however, clearly apparent: Compromised energy metabolism of the cells leads to swelling and changes in the sodium:potassium ratio. In addition even the most healthy slices contain 25% dead cells. The radiotracer methods give information about different compartments in the whole tissue; therefore one can not correct readily for the compromised metabolic function. In contrast, sophisticated electrophysiological methods are now available for the measurement of extracellular and intracellular functions in brain slices (see chapters by Nicholson and Rice and by MacVicar and O’Beirne in this volume). Here, individual cells can be tested before the experiment if they conform to certain standards. The inevitable cell swelling that creates certain difficulties for the interpretation of compartmental fluxes seems not to compromise the electrophysiological function of individually tested neurons. With the use of homogeneous primary cultures of neural cells, an alternative system with fewer limitations became available. Brain slices have, however, gained new popularity in measuring energy metabolism and electrical transmission in certain pathological states induced by anoxia, hypoglycemia, and ischemia (see Lipton and Whittingham, 1984). In this context, analyses of fluid compartments and ion fluxes are quite frequently used to give a more complete picture about events involved in such states.