The ultrastructure of the epithelial cells of Reissner's membrane (membrana vestibularis) in the guinea-pig is described following vascular perfusion with glutaraldehyde of live, anaesthetised and artificially respirated animals. Postfixation in a solution containing OsO4 and potassium ferricyanide revealed a well-developed tubulocisternal endoplasmic reticulum, not previously described, the continuity of which has been mapped by serial sectioning and reconstruction. Large disc-shaped subsurface cisternae lining the cell membrane, but separated from it by a space approximately 10 nm wide, are in continuity with the smooth endoplasmic reticulum, forming an elaborated transcellular canalicular pathway. This structure is compared to that found in solute-transporting epithelia, e.g., renal proximal tubule, gall bladder, small intestine and choroid plexus. The fixation method used in the present study is compared to other techniques used for preservation of Reissner's membrane. Each epithelial cell of Reissner's membrane is endowed with one kinocilium, one to four multivesicular bodies, and a number of intercalated bodies. The functional significance of the canalicular pathway is discussed.