Intracellular pH (pHi) and the mechanisms of pHi regulation have been investigated in cultured microglial cells from mouse brain using the pH-sensitive fluorescent dye 2',7'-bis-(2-carboxyethyl)-5-(6)-carboxyfluorescein (BCECF). Cells were acidified by a pulse of NH4+ (4-5 min; 20 mM) and the subsequent pHi recovery from an acidification was studied. In HCO3(-)-free saline, pH regulation was dependent on extracellular [Na+] and sensitive to amiloride, indicating the involvement of the Na+/H+ exchanger. In HCO3(-)-containing solution 2 mM amiloride slowed but did not block pHi recovery; the recovery however was dependent on extracellular [Na+] and sensitive to 0.3 mM DIDS, suggesting the presence of Na+/HCO3 cotransporter and/or Na(+)-dependent Cl-/HCO3-exchanger. The involvement of a Na-dependent Cl-/HCO3-exchanger was inferred from the observation that removal of Cl- or application of 1 mM furosemide decreased but did not block the recovery rate. Increasing [K+]0 resulted in an alkalinization by a process that was neither HCO3- nor Na(+)-dependent, nor DIDS- and amiloride-inhibitable. In conclusion, microglial cells express a distinct set of pH regulatory carriers which control for a defined level of pHi. An increase in [K+]0 can offset this level.