The purpose of these studies was to look for evidence of a pH-sensitive modifier site on the Cl-HCO3 exchanger on the brush-border membrane in rabbit ileum utilizing membrane vesicles. When internal pH and HCO3 were increased, Cl uptake was stimulated in a sigmoidal fashion consistent with a modifier effect. Increasing internal HCO3 alone did not have a similar effect, and increasing pH alone in the absence of HCO3 resulted in very little uptake of Cl. These results suggested that OH was a poor substrate for the exchanger but that it "activated" the transport of HCO3. Further evidence for this hypothesis was provided by the observation that increasing internal pH also stimulated Cl-Cl exchange. Altering the membrane potential with K and valinomycin had no effect on Cl uptake at high internal pH, suggesting no change in the 1:1 coupling ratio for Cl-HCO3 exchange at high internal pH. These studies provide evidence that there is an internal pH-sensitive modifier site on the Cl-HCO3 exchanger.