These studies seek to define the gene expression of proteoglycans and collagens in the developing hypertrophic phase of an experimental model of osteoarthritis (OA). Total RNA was extracted from articular cartilage of nonoperated and operated dog knees 10 weeks after induction of OA by transection of the anterior cruciate ligament. The relative amounts of mRNA for type II collagen and the core proteins for the aggrecan, biglycan, decorin, and fibromodulin were analyzed by Northern blotting. Total RNA in OA vs nonoperated knees was statistically significantly elevated 2.5x, the mRNA for type II collagen was elevated 8x, aggrecan 2x, biglycan 4x, fibromodulin 2x. The level for decorin was increased 1.6x, but this difference was not statistically significant. Chondrocytes respond actively to joint injury. Gene expression of proteoglycans and type II collagen is discoordinate in early experimental OA, and may contribute to the development of cartilage abnormalities.