Osteoarthrosis is characterized in the early stages by degradation of articular cartilage matrix. Clinical, radiological, and pathological studies have failed to reveal the factors which initiate the breakdown of cartilage and are not applicable to detailed sequential studies of the affected tissues at all stages in the disease. Therefore animal experiments have been employed to provide more information on degradation and repair process in cartilage. These studies have demonstrated: 1) Matrix protection and induced repair of mature articular cartilage by the use of oral aspirin after lacerative injury. 2) Establishment by the intra-articular injection of the plant enzyme papain of a model of osteoarthrosis in the rabbit hip which mimics human osteoarthrosis and is suitable for further experimental studies. 3) A proliferation of mature articular cartilage chondrocytes in response to loss of matrix, which indicates a latent repair capacity. 4) Repair of extensively damaged hip joints after femoral osteotomy by increased formation of subchondral new bone and formation of fibrocartilage on the articulating surfaces. These tissue repair processes are associated with an increase in vascularity of the femoral head and acetabulum produced by the osteotomy. 5) Succesful transplantation as allografts in both normal and arthrotic rabbit knees of aggregates of epiphysial chondrocytes isolated from their matrix. This method of joint surface replacement may have clinical applications.