Carbonic anhydrase was detected histochemically in the guinea pig endolymphatic sac. The enzyme reaction was positive in both light and dark epithelial cells. In the former, the reaction product was found in the cytoplasm, especially around the intracytoplasmic vesicles and vacuoles. Reaction product was found in the basolateral infoldings as well. The dark epithelial cells also displayed carbonic anhydrase activity in the cytoplasm as well as in the lysosome-like bodies. It is suggested that this enzyme may be involved in (1) ionic or fluid regulation of the endolymph, (2) otoconia metabolism, and (3) phagocytotic activities.