There are several substances whose ISF concentrations must be kept within narrow limits to ensure correct neuronal function. Na+, Cl− and K+ are good examples. Regulation of Na+ and Cl− amounts and concentrations is inextricably linked to the control of extracellular fluid volume and intracranial pressure and is outside the scope of this review (for some discussion see [41]). The control of K+ and HCO3− ISF concentrations was considered in [4]. The following sections consider the general principles and the control of ISF concentrations of CO2 and glucose.