The endolymph in the endolymphatic sac (ES) is acidic (pH 6.6–7). Maintaining this acidic lumen is believed to be important for the normal function of the ES. The acid-base regulation mechanisms of the ES are unknown. Here we investigated the expression patterns of acid-base regulators, including vacuolar (v)H⁺-ATPase (proton pump), carbonic anhydrase (CA) II, and pendrin in the murine ES epithelium by immunohistochemistry (IHC) and compared their expression patterns by double immunostaining. We found that pendrin and vH⁺-ATPase were co-localized in the apical membrane of a specific type of ES epithelial cell. Pendrin- and vH⁺-ATPase-positive cells also expressed cytoplasmic CA II. Co-expression of pendrin, vH⁺-ATPase, and CA II in the same subgroup of ES cells suggests that this specific type of ES cell is responsible for the acid-base balance processes in the ES and pendrin, vH⁺-ATPase, and CA II are involved in these processes.