Double-barrelled ion-sensitive micro-electrodes were used to measure the changes of the intracellular activities of Cl-, K+, and Na+ (aiCl, aiK, aiNa) in neurones of isolated rat sympathetic ganglia during the action of gamma-aminobutyric acid (GABA). The membrane potential of some of the neurones was manually 'voltage clamped' by passing current through the reference barrel of the ion-sensitive micro-electrode. This enabled us to convert the normal depolarizing action of GABA into a hyperpolarization. A GABA-induced membrane depolarization was accompanied by a decrease of aiCl, aiK and no change in aiNa, whereas a GABA-induced membrane hyperpolarization resulted in an increase of aiCl, aiK and also no change in aiNa. GABA did not change the free intracellular Ca2+ concentration, as measured with a Ca2+-sensitive micro-electrode, whereas such an effect was seen during the action of carbachol. pH-sensitive electrodes, on the other hand, revealed a small GABA-induced extracellular acidification. The inward pumping of Cl- following the normal, depolarizing action of GABA required the presence of extracellular K+ as well as Na+, whereas CO2/HCO3--free solutions did not influence the uptake process. Furosemide, but not DIDS, blocked the inward pumping of Cl-. In conclusion, our data show that only changes in intracellular activities of K+ and Cl- are associated with the action of GABA. Furthermore, they indicate that a K+/Cl- co-transport, and not a Cl-/HCO3- counter-transport, may be involved in the homoeostatic mechanism which operates to restore the normal transmembrane Cl- distribution after the action of GABA.