In order to study the mechanism of H+ and HCO3- transport in a HCO3- secreting epithelium, pancreatic ducts, we have measured the intracellular pH (pHi) in this tissue using the pH sensitive probe BCECF. We found that exposures of ducts to solutions containing acetate/acetic acid or NH4+/NH3 buffers (20 mmol/l) led to pHi changes in accordance with entry of lipid-soluble forms of the buffers, followed by back-regulation of pHi by duct cells. In another type of experiment, changes in extracellular pH of solutions containing HEPES or HCO3-/CO2 buffers led to significant changes in pHi that did not seem to be back-regulated efficiently by duct cells. The sensitivity of pHi to the inhibitor HOE 694 and to changes in Na+ gradients, indicate that the Na+/H+ exchanger is present in this epithelium. Similarly, the sensitivity to Cl- and HCO3- gradients indicated the presence of the Cl-/HCO3- exchanger. Under some conditions, these exchangers can be invoked to regulate cell pH.