The localization of Ca2+-ATPase activity was examined ultracytochemically in the endolymphatic sac (ES) of the 1-day-old chick. The reaction products showing Ca2+-ATPase activity were localized along the microvilli and the apical cell membranes of the epithelial cells of the ES. The lateral cell membranes also showed mild activity. These reactions were completely abolished when either Ca2+ or ATP was omitted from the incubation medium. The results suggest that Ca2+-ATPase plays a significant role as a Ca2+ pump for regulation of the Ca2+ concentration in the endolymph of the ES.