The chinchilla round window membrane consists of three layers: (1) an outer epithelial layer facing the middle ear cavity with tight junctions and numerous desmosomes, (2) a middle connective-tissue layer consisting of fibroblasts, fibrocytes, collagen, elastin, capillaries, and myelinated and unmyelinated nerves, and (3) an inner cellular layer with thin cytoplasmic processes containing numerous micropinocytotic vesicles and dark granules similar in form to alpha-type glycogen. Studies of the fine structure of the normal round window membrane in animals are prerequisites to understanding the role of the round window membrane in both normal function and in diseased states such as otitis media.