The ultrastructure of the guinea pig endolymphatic sac was studied, using the ruthenium red staining technique. The dye stained the apical cell surface coat and the homogeneous substance in the luminal space of the endolymphatic sac, when introduced from the luminal side of the epithelium. It is suggested that the surface coat and homogeneous substance may play an important part in the endolymph regulatory mechanism in the endolymphatic sac. When ruthenium red was introduced from the subepithelial side, the basolateral surface of the epithelial cells usually became brightly stained in the absence of staining of the apical cell surface, due to the presence of the tight junctions. In some instances, however, the dye penetrated beyond the level of the tight junctions. Pinocytotic vesicles and larger vacuoles in the epithelial cells were also sometimes stained, both apically and near the lateral cell surface. These findings suggest that endolymph efflux mechanisms in the endolymphatic sac may involve the combined actions of a paracellular and transepithelial flow as well as a transcellular, vacuolar bulk flow.