We studied the mechanisms regulating intracellular pH (pH_i) in cultured rat astrocytes and neurons, with particular reference to the influence of extracellular pH (pH_e) on these mechanisms, using microspectrofluorometric monitoring from single cells, loaded with the pH-sensitive fluorophore BCECF. The pH regulatory mechanisms differ between neurons and astrocytes. The experimental data suggest the presence of a Na⁺/H⁺ and a Na⁺-mdependent HCO₃ ^-/C1^- exchanger in both types of cells, while astrocytes, in addition, utilise a Na⁺-dependent HCO₃ ^-/C1^- exchanger for regulating acid transients. In both cell types the pH regulatory mechanisms are strongly dependent on pH_e. Thus, at pH_e 6.85 or below, there was no recovery of pH_i. Steady state pH_i was also strongly dependent on pH_e, in both astrocytes and neurons. The pH_j recovery following normalisation of pH_e was very rapid, (indicating that a prolonged exposure to a low pH stimulates pH regulating mechanisms), and was inhibited by 4,4’-diisothiocyanatostilbene-2,2’disulphonic acid (DIDS) and amiloride, or in the absence of Na⁺. The results challenge the concept of a H⁺-regulatory site solely at the internal side of the exchanger regulating pH_i to a constant value.