1. Intracellular pH (pHi) and the mechanism of pHi regulation have been investigated in cultured oligodendrocytes from mouse spinal cord using double-barrelled neutral-carrier H+-selective microelectrodes. The distribution of H+ was not in electrochemical equilibrium. The pHi was more alkaline than the pH of the bathing medium (pHo), namely 7.5 at pHo 7.2 at 7.6 at pHo 7.4. 2. Removal of HCO3- from the bathing medium reduced the steady-state pHi by 0.4 units. An increase in extracellular K+ caused, with a delay, an increase in pHi. A decrease in pHo to 6.2 caused an acidification of pHi by 0.5 units. 3. The pHi regulation was studied by applying and subsequently removing NH4+ which resulted in an acidification of the cell. The subsequent recovery of pHi could then be analysed. The recovery from an acidification by 1 pH unit lasted 3-10 min. In HCO3- -free solution pHi recovery was slowed. 4. In HCO3- -free solution pHi recovery was completely blocked when either Na+ was removed or when amiloride was applied indicating an exclusive activation of the Na+-H+ exchanger. 5. In the presence of HCO3-, removal of Na+ also completely blocked pHi recovery. When Na+ was readded, pHi recovered. In HCO3- -containing solution amiloride slightly slowed, but did not block pHi recovery. 6. Removal of Cl- or application of SITS, DIDS or furosemide, blockers of Cl- -coupled transport mechanisms, did not affect the pHi recovery in the presence of HCO3-. 7. In conclusion, oligodendrocytes possess two mechanisms regulating pHi, a Na+-H+ exchanger and a Na+-HCO3- co-transporter while the latter is clearly more potent. It follows that pHi regulation of oligodendrocytes is dependent on the transmembrane Na+ gradient and is strictly separated from regulation of internal Cl-.