Experimental osteoarthrosis of the knee joint was induced in rabbits using a polyethylene binding brace. A 1% solution of sodium hyaluronate (HA) of molecular weight 1.73 × 106 (HA-173) was administered intraarticularly at a dosage of 0.1 ml/kg body weight twice a week. Changes in articular cartilage and synovial membrane, and joint contracture were compared with a saline control group at intervals throughout a six-week period of immobilization. In all groups, articular cartilage degeneration and joint contracture progressed with time but were inhibited by the administration of HA-173. Additionally, the results of a 1% solution of HA of molecular weight 9.8 × 105 (HA-98), a 1% solution of HA of molecular weight 2.02 × 106 (HA-202), and saline, each administered twice a week for five weeks at a dose of 0.1 ml/kg body weight, were compared. It was clear that the articular cartilage degeneration was significantly inhibited by HA as compared with saline, and that the effect was more potent with the higher molecular weight, HA-202, than with HA-98. Furthermore, joint contracture was inhibited by HA, and the effect was more potent with the higher molecular weight, HA-202, than with HA-98.