Extracellular HCO3- hyperpolarizes the intracellular potential and makes the aqueous medium negative with respect to the stromal surface of the rabbit ciliary epithelial syncytium. The bases for these observations have been unclear. We have been studying the bicarbonate-induced hyperpolarization (BIH) with sustained intracellular recordings for periods as long as 1-2 hrs.The BIH was observed [6.0±0.4 mV (mean±SE, N=22)] even when the external pH was clamped constant by appropriately changing the CO2tension. External HCO3was required since aeration with CO2at low external pH did not replicate the BIH. DIDS [4,4'-diisothiocyano-2,2'-disulfonic acid] did not abolish the effect. The hyperpolarization is unlikely to reflect the pH dependence of K+channels alone, since the effect was not reduced by either 2 mM Ba2+alone or 2 mM Ba2+together with 50-100μM quinidine. The BIH depends directly or indirectly on external Na+, since the sign of the polarization response was reversed either by replacing Na+with N-methyl-D-glucamine or by blocking the Na+,K+-exchange pump with 50-100μM ouabain. Replacement of external CI−with NO−3or application of the Cl−-channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate] depolarized the membrane and reversed the sign of the BIH.The response of the ciliary epithelium to HCO3- is complex and may arise from several mechanisms. We suggest that one important element is an anion channel whose conductance is reduced by bicarbonate and whose reversal potential is indirectly dependent on the operations of the Na+,K+-pump and a Cl−-linked symport.