The epithelium of the mammalian urinary bladder is an effective barrier to the free flux of urea, water and small charged ions between urine and tissue fluids. In this paper we present evidence to show that this permeability barrier passively depends on the structure of the unusual, thick, rigid membrane which limits the urinary surface of the epithelial cells. The normal structure of this membrane is unique. Thickened plaques, composed of a hexagonal lattice of dodecameric subunits which extend through the depth of the membrane, are separated by thinner, unstructured, narrow bands. The regular structure of the lattice may be disrupted or distorted by treatment with urea, guanidine hydrochloride or sodium dodecyl sulphate. Chemically the membrane is also remarkable. Its lipid component contains cerebroside, which apparently has an important effect in lowering water permeability. This effect is reinforced by cholesterol, which may be important for the maintenance of a condensed lipid layer during the mechanical stresses of bladder contraction and dilation. The protein component also is unusual in having a high proline content, and the significance of this is discussed in the context of membrane structure and function. At the moment it is not possible to relate the marked subunit structure of the membrane to its protein component although the large particle mass mucoprotein compound, as yet uncharacterized, may be located in the thick, densely staining, negatively charged surface lamina of the membrane. This membrane is thus remarkable in both its structure and its function. These properties in turn must depend on complex interactions between its lipid and protein components which are themselves unusual.