Casting of the right knee (stifle) joints of young beagle dogs for 11 weeks caused up to 48% reduction in the glycosaminoglycan (GAG) concentration of the uncalcified articular cartilage, as assessed by a new microspectrophotometric method. The GAGs were depleted mainly in the superficial zone of the cartilage. Although the thickness of the uncalcified cartilage was not decreased, the calcified cartilage under the tidemark was thinned by 6-25% at the femoral condyles. The increased weight-bearing in the limb opposite the one in the splint caused uncalcified cartilage thickness to be augmented by 19% and GAG concentration by 25-35% in the intermediate, deep, and calcified zones of the summits of the femoral condyles; the changes were smaller in other, less loaded parts of the joint. It is concluded that in young dogs, increased weight-bearing augments local proteoglycan content of the articular cartilage matrix, while unloading reduces it.