Topographically, there are both morphological and biochemical differences in the articular cartilage of the tibial plateau of normal adult dogs when the cartilage covered by the meniscus is compared with that more centrally placed and not covered by meniscus. Histologically, differences are present in the surface morphology, in intra- and extracellular lipid content, and in the morphology of the mineralization front. Electron microscopy shows, in the covered cartilage, variability in collagen fiber size, with evenly spaced fibers apparently randomly distributed and an orderly relationship between the proteoglycans and collagen, whereas in the uncovered area, the collagen is aggregated into bundles and appears to be dissociated in large part from the proteoglycans. The most striking feature in the biochemistry of the two regions is an increased water content in the uncovered cartilage, as compared with the covered. In addition, there is an increased amount of proteoglycans that can be extracted in the uncovered cartilage. The heterogeneity of the cartilage on the tibial plateau should be taken into account when considering both the histologic and biochemical variations found in osteoarthritic cartilage; and when reflecting on the pathogenesis of osteoarthritis.