Ultrastructural studies of the round window membrane in infants revealed that the membrane is made up of three layers. The relatively wide outer epithelial layer consists of 3–4 cell layers and is formed by cuboidal and round cells. The outer two or three cell layers contain mainly rough endoplasmic reticulum and secretory granules, indicative of a secretory function of the epithelial cell layer of middle ear mucosa. Tonofilaments occur mainly in the more electron-dense epithelial cell layers which makes their mechanical role more probable. The connective tissue layer is relatively narrow and contains fibrocytes, much collagen and relatively few elastic fibres. The inner epithelial layer is built up of elongated cells containing rough endoplasmic reticulum cisternae in their cytoplas