Intracellular pH (pHi) of acid-secreting cells was measured in intact gastric fundus mucosa of Rana esculenta with double-barrelled pH microelectrodes. Tissues were mounted, serosal side up, between two half chambers and individual cells were impaled after microsurgical removal of the serosal muscle layer. Transepithelial potential difference (Vt) and resistance (Rt) as well as serosal cell membrane potential (Vs) and pH i were continuously recorded at rest (0.1mmol/1 cimetidine) or during stimulation (0.5mmol/1 histamine). During chamber perfusion with HCO~-/CO2-buffered Ringer solution of pH o = 7.36, V t and R t were -21.7, SD+6.0 mV and 229_+83 f2 cm 2 (n = 17) while V s and pH i averaged -57.3_+6.9 mV and 7.4+0.11 (n = 25). The latter value is considerably more alkaline than all recent pHi measurements obtained with microspectrofluorometric techniques on isolated cells, glands or intact tissue. The difference may in part be explained by use of HCO3-free solutions in most of the previous studies because we observed that such solutions decrease pH i to 6.89+0.18 (n = 4). Again, in contrast to recent literature, application of histamine in HCO3/COz-buffered solution led to further transient alkalinization by 0.12+0.05 pH unit (n = 8). Since in accidental punctures of the gastric gland lumen we noticed that H + secretion only began approximately 5 min after histamine application, we conclude that the histamine-induced initial alkalinization does not reflect stimulation of the H +/K + ATPase pump. Alternatively, it may result from histamine-induced activation or inactivation of other ion transporters, one possibility being activation of basolateral Na +/H + and C1-/HCO3 exchangers.