Objective. To quantify the gene expression of aggrecan core protein and type II collagen in an experimental animal model of osteoarthritis (OA). Methods. Total RNA was extracted from the articular cartilage of unoperated knee joints and from OA joints produced by anterior cruciate ligament transection. The relative amounts of type II collagen and aggrecan core protein messenger RNA (mRNA) were evaluated by Northern blot analysis. Results. Total RNA was elevated 2.5 times, aggrecan mRNA was elevated 2 times, and type II collagen mRNA was elevated 8 times, in OA knees compared to unoperated controls. Conclusion. Chondrocytes are activated metabolically in response to joint injury. Discoordinate gene expression of aggrecan and type II collagen is characteristic of early experimental OA, and we speculate that it may contribute to its pathogenesis.