Resting or basal intracellular pH (pHi) measured in cultured human syncytiotrophoblast cells was 7.26+/-0.04 (without HCO3-) or 7.24+/-0.03 (with HCO3-). Ion substitution and inhibitor experiments were performed to determine whether common H+-transporting species were operating to maintain basal pHi. Removal of extracellular Na+ or Cl- or addition of amiloride or dihydro-4,4'-diisothiocyanatostilbene-2,2'-disulfonate (H2DIDS) had no effect. Acidification with the K+/H+ exchanger nigericin reduced pHi to 6.25+/-0.15 (without HCO3-) or 6.53+/-0.10 (with HCO3-). In the presence of extracellular Na+, recovery to basal pHi was prompt and occurred at similar rates in the absence and presence of HCO3-. Ion substitution and inhibition experiments were also used to identify the species mediating the return to basal pHi after acidification. Recovery was inhibited by removal of Na+ or addition of amiloride, whereas removal of Cl- and addition of H2DIDS were ineffective. Addition of the Na+/H+ exchanger monensin to cells that had returned to basal pHi elicited a further increase in pHi to 7.48+/-0.07. Analysis of recovery data showed that there was a progressive decrease in DeltapH per minute as pHi approached the basal level, despite the continued presence of a driving force for H+ extrusion. These data show that in cultured syncytial cells, in the absence of perturbation, basal pHi is preserved despite the absence of active, mediated pH maintenance. They also demonstrate that an Na+/H+ antiporter acts to defend the cells against acidification and that it is the sole transporter necessary for recovery from an intracellular acid load.