In recent years, the rat has attracted increasing interest as an experimental animal in otological research. In the present study, the structure of the rat tympanic membrane (TM) is compared with that of a traditional experimental animal, the guinea pig, and with the human TM. The normal guinea pig TM consists mainly of a single layer of coarse fiber bundles, in contrast to the more solid layers of radial and circular fibers in man and rat. The investigation reveals similar architectural features in the rat and human TM, whereas the guinea pig TM differs structurally.