The use of ion-sensitive microelectrodes enabled us to follow the dynamic changes in extracellular pH (pHe) together with those in the extracellular concentration of some biologically important ions, particularly K+ and Ca2+. Activity-related changes in pHe were studied in isolated spinal cords of frogs and in spinal cords of rats in vivo. Repetitive electrical stimulation of an afferent input led either to triphasic alkaline-acid-acid changes (90% of frogs) or to triphasic alkaline-acid-alkaline changes (10% of frogs and rats) with the greatest changes in the lower dorsal horns. The transient acid shift by as much as 0.15-0.25 pH units is dominant and builds up during the stimulation. The changes in pHe were also found in response to various adequate stimuli applied to the skin on the hind limb. Using specific inhibitors of Na+/H+ exchange, K+-Cl- co-transport, Cl-/HCO3- exchange, the Na+/K+ pump and carbonic anhydrase, we found pHe homeostasis to be impaired and stimulation-induced changes in pHe decreased. We conclude that the pHe changes evoked by electrical or adequate stimulation of an afferent input are not determined by changes in extracellular strong ion concentration differences due to accumulation of lactate, since we found no effect of NaF, a metabolic blocker of lactate production. However, lactate accumulation has been demonstrated during seizures, spreading depression and anoxia. Recently, it has been recognized that the observed pHe changes can affect permeability of membrane ionic channels, neuronal excitability and glial cell function.