Histochemical, x-ray analytical and scanning and transmission electron microscopical procedures have been utilized to determine the chemical nature, physical appearance and attachments of the tectorial membrane in normal rats and to correlate these results with biochemical data on protein-carbohydrate complexes. Additionally, pertinent histochemical and ultrastructural findings in chemically sympathectomized rats are considered. The results indicate that the tectorial membrane is a viscous, complex, colloid of glycoprotein( s) possessing some oriented molecules and an ionic composition different from either endolymph or perilymph. It is attached to the reticular laminar surface of the organ of Corti and to the tips of the outer hair cells; it is attached to and encloses the hairs of the inner hair cells. A fluid compartment may exist within the limbs of the "W' formed by the hairs on each outer hair cell surface. Present biochemical concepts of viscous glycoproteins suggest that they are polyelectrolytes interacting physically to form complex networks. They possess characteristics making them important in fluid and ion transport. Furthermore, the macromolecular configuration assumed by such polyelectrolytes is unstable and subject to change from stress or shifts in pH or ions. Thus, the attachments of the tectorial membrane to the hair cells may play an important role in the transduction process at the molecular level.The present investigation is an outgrowth of a prior study of the effects of chemical sympathectomy upon the structures of the inner ear. None of the findings of this prior study were more interesting than those concerning the internal structure and the attachments of the tectorial membrane. In chemically sympathectomized rats, the filamentous organization of the membrane was often remarkably evident ultrastructurally, and the membrane showed a proclivity for remaining attached to the tips of the hairs of the outer hair cells. These results raised questions concerning the chemical nature and attachments of the tectorial membrane in untreated rats, and whether or not these properties had been altered by chemical sympathec tomy.A search of the literature showed that the answers to the questions posed were not to be found there for, although the membrane has been extensively studied from the time of Corti (1851) to the present, the origin, attachments, chemical nature, physical appearance and functions AM. J. ANAT., 139: 449482.of the tectorial membrane remain matters of dispute. As will be made evident in the Review of the Literature which follows, one can find support for almost any interpretation of the structure of the tectorial membrane or its degree of attachment to the organ of Corti. Although the tectorial membrane is generally accepted to be rich in acid mucopolysaccharide, this point is also a matter of some contention.It was decided, therefore, to undertake histochemical and scanning electron microscopical research which might provide new information on certain of the controversial a...