Mutations of the anion exchanger pendrin are responsible for Pendred syndrome, an autosomal recessive disease characterized by deafness and goiter. Pendrin is highly expressed in kidney collecting ducts, where it acts as a chloride/bicarbonate exchanger and thereby contributes to the regulation of acid-base homeostasis and blood pressure. This study aimed to characterize the intrinsic properties of pendrin. Mouse pendrin was transfected in HEK293 and OKP cells; its activity was determined by monitoring changes in intracellular pH induced by variations of transmembrane anion gradients. Combining measurements of pendrin activity with mathematical modeling, we found that its affinities for Cl^-, HCO3 ^- and OH^- vary with intracellular pH, resulting in increased activity at low intracellular pH. Pendrin maximal activity was also stimulated at low extracellular pH, suggesting the presence of intra and extracellular proton regulatory sites. We identified 5 putative pendrin glycosylation sites, only two of which are used. Mutagenesis-induced disruption of pendrin glycosylation did not alter its cell surface expression, polarized targeting to the apical membrane and basal activity, but fully abrogated its sensitivity to extracellular pH. The hereto unknown regulation of pendrin by external pH may constitute a key mechanism in controlling ionic exchanges across the collecting duct and inner ear.