This review examined the relation between interstitial hydraulic permeability and chemical composition, using porous matrix theory, and assessed the view that interstitial conductivity is governed by GAG concentration. Conductivity correlates negatively with both GAG and collagen concentration over a wide variety of tissues. Conductivities of GAG matrices in vitro, coupled with other quantitative considerations, indicate, however, that no single class of fixed fibrous element exists at a sufficiently high concentration to account by itself for the low conductivity in most tissues. It seems that the low interstitial conductivity arises from the combined interactive effects of three main classes of fibrous element: collagen fibrils, GAG and proteoglycan core protein. In most cases the proteoglycan complex plays a major role which is significantly amplified, however, by the collagen fibril network.