The three-compartment model of brain acid-base regulation postulates that under circumstances of changing function or disease, hydrogen ion concentrations may differ considerably in the interstitial space (ISS), the neurons and the glial cells. During hyperglycemia plus profound ischemia, for example, direct measurements by microelectrodes followed by intracellular HRP staining show that intraglial pH can fall transiently as low as 3.9, although more often the nadir drops to the 4.5-5.5 range. Concurrently, ISS-pH and, by calculation, neuronal pH fails to and remains constant (but not necessarily the same) at pH 6.2. By contrast, during spreading depression, ISS and intraglial pH at first move rapidly and transiently in opposite directions, ISS [H+] rising, intraglial falling. These two then gradually stabilize, whereas neuronal pH remains substantially more steady and near normal, shifting only minimally from resting baseline levels over several minutes' time. Similar but less pronounced effects follow direct electrical stimulation. The net change represents complex biophysical transmembrane and buffering mechanisms that appear to guard neuronal homeostasis. Studies carried out on embryonic rat forebrain neurons and glia show that these cells have considerably different vulnerabilities to extracellular acidity depending on the anionic nature of the acid in the bathing medium. In cultures to which HCI was added to the medium, neurons and neuronal processes almost all survived ten minute exposures to pH 3.8, whereas glial cells succumbed after ten minute exposures at pH not lower than 4.2.(ABSTRACT TRUNCATED AT 250 WORDS)