The localization of carbonic anhydrase (CA) in mouse and guinea pig endolymphatic sac (ES) was determined by immunohistochemistry. In the distal and intermediate portions of the ES of both species, epithelial cells reacted with anti-CA antiserum. All epithelial cells in the distal portion stained for CA. In the intermediate portion, immunoreactivity was prominent in about half the epithelial cells, but less intense in the remaining epithelial cells. The proximal portion of the ES lacked immunoreactivity. CA may play a crucial role in ES electrolyte transport.