The ultrastructure and barrier properties of the endolymphatic duct (ED) were examined by light and electron microscopy. ED epithelial cells were classified into two types: type I and type II duct cells. The type I duct cells were cuboidal or low columnar and were characterized by a convex apical surface and a few basal processes. The type II duct cells were squamous and were characterized by a flat apical and basal membrane, many small vesicles and a number of small pits along the basal membranes. After electrophoretic horseradish peroxidase (HRP) injection into the ED lumen, no HRP uptake into ED epithelial cells of either type was observed. There was no reaction product either in the lateral intercellular spaces beyond the tight junctions between ED epithelial cells or in ED subepithelial tissues. The ED epithelial cells were considered to play no active role in apical macromolecular absorption and to be impermeable to intraluminal macromolecules.