AIM: To investigate the effects and mechanisms of intraarticular injection of doxycycline on experimental osreoarthritis in rabbits. METHODS: An animal model of osteoarthritis in knee of rabbits was established by intraarticular injection of papain. 2 mg or 4 mg of doxycycline was injected (intraarticular) once a day in four weeks. The degeneration of articular cartilage, Mankin s marks of the cartilage tissue, the expression of MMP-13, the release of nitricoxide (NO) and the activity of total nitricoxide synthase (NOS) and inducible nitricoxide synthase (iNOS) of the joint fluid were tested subsequently . RESULTS: In the osteoarthritis model group, the Mankin s marks and the expression of MMP-13 observably increased, and spectrophotometric analysis showed the high concentration of the release of NO and high activity of NOS and iNOS in the joint fluid (P