The normal ultrastructure and the phagocytotic activity of the endolymphatic sac in guinea pigs were studied. The epithelial cells of the endolymphatic sac were classified into two types, (1) One is characterized by numerous mitochondria, many small vesicles and well-developed microvilli at the apical surface. (2) The other is characteristic for poorly developed cytoorganelles and a few microvilli and numerous intracellular filaments. In addition the intermediate type between the two were also seen. India ink particles injected into the endolymphatic space, were taken up not only by the free floating cells but also by the epithelial cells.
