The vestibular membranes (REISSNER's membranes) of the normal adult guinea pigs, rats, mice, bats, rabbits and cats were observed with the electron microscope. The specimens were fixed with 1% OsO4 adjusted to pH 7.4 with phosphate or veronal acetate buffer, and embedded either in a mixture of styrene and n-butyl methacrylate (1:1) or epoxy resin. Thin sections were made with PORTER-BLUM microtome using glass knives, and stained with uranyl acetate. Microscopy was performed with either HU-10 or JEM-T6 electron microscope, and obtained the following results:1. The vestibular membrane is divided by a single basement membrane into the ectodermal epithelium facing the cochlear duct and the mesenchymal epithelium facing the scala vestibule Either epithelium is simple monolayered, but the former is continuous while the latter is porous (discontinuous).2. Epithelial cells of the cochlear duct are flat in the case of guinea pigs, rabbits and cats, and contain oval nuclei whose long axes are parallel to the basement membrane. In the case of rats, mice and bats, the epithelial cells are cuboidal and the nuclei are spherical.The free surface of the cells facing the endolymph is provided with many microvilli, which are numerous at the region near the intercellular junctions. The lateral cell surface adjoining the neighbouring cells is not usually straight, but forms an elaborate interdigitations especially at the basal half or one-third of the lateral cell wall.Development of the interdigitation may vary widely according to the difference of animal species, and also differs from cell to cell even in the same organelle. In mice, rats, rabbits and bats, the intercellular interdigitations are well developed, but in the bat and cat they are poorly developed and simple curvatures are observed near the base. The intercellular space is usually narrow and uniform in thickness, but in a region where the interdigitations are remarkable the space may be dilated at the tip of the curvature. The terminal bar seal up the interecellular space at the luminal edge of the lateral boundaries, and desmosomes can be observed in many places of the intercellular cell boundaries.In the epithelial cells of the rats and mice cochlear ducts, small vesicles, tubules or sacs are aligned into a row which connect the lateral to the basal cell surface. It seems likely that the row of the vesicles is a section through a turtuous tubule or tubules which sometimes dilate into sacs. In the rabbit, remarkable structures are observed at the basal part of the epithelial cell. They probably be produced by extension of cytoplasmic processes fitted in the invaginated cavity of the basal infolding. The processes may be extended from the own cell body or from the neighbouring cells. Therefore, such a complicated unity of the infoldings and interigitations is called an islands-like structure.Along the basal all surface, various types of infoldings of the plarma membrane can be observed. In the guinea pig, the basal surface is almost smooth with simple small indentations; in the rat and bat are observed somewhat complicated invaginations or infoldings which are reminiscent of the basal infoldings of the HENLE's loop of the mammalian renal tubules; besides these in the rat cochlear epithelium small tubules with a turtuous course are found in the region near the nucleus and clearly open to the basal surface with several orifices; in the rabbit extremely complicated infoldings with islands-like structures are seen in the basal cytoplasm; while in the cat relatively large indentations and rather simple islands-like structures can be observed.In the cytoplasm, many small vesicles are diffusely scattered, and along the cell periphery not only under the free surface but also lateral and basal surfaces near interdigitations and infoldings many pinocytotic vesicles are observed.