Na,K-ATPase subunit isoform expression was studied by immunocytochemistry in the guinea pig endolymphatic sac, using subunit isoform-specific polyclonal antibodies. Epithelial cells of the guinea pig endolymphatic sac were observed to contain the α1- and β2-subunit isoforms, and to a lesser extent the β1-subunit isoform, of Na,K-ATPase. The α1- and β2-subunit isoforms of Na,K-ATPase have been observed previously in other ion and fluid transporting regions of the membranous labyrinth, e.g., stria vascularis and vestibular dark cells. Combined data indicate that the α1-, β2-form of Na,K-ATPase plays a role in the microhomeostasis of endolymph. The α1β2 Na,K-ATPase subunit isoform combination is different from typical ion and fluid transporting tissues, e.g., kidney and colon, and may reflect distinctive characteristics of inner ear Na,K-ATPase.