Because of the pathological similarities between collagen induced arthritis (CIA) and rheumatoid arthritis, the CIA model has been the subject of extensive investigations. This study initiated to evaluate the pathologic lesions of induced arthritis by collagen in a total of 50 female rats (Rattus norvegicus.). Arthritis was induced by two intradermal injections (14 days interval) of type 2 bovine collagen solution and Freund's complete adjuvant (FCA) into the tail and right leg paw of 40 rats. Severity of arthritis in affected leg was represented in grade 1 to 4. Three weeks after beginning of the experiment, 22, 12, 5 and 1 affected rats were clinically diagnosed with 1, 2, 3 and 4 grades of arthritis respectively. Rats were intradermally injected into the tail showed only slight and moderate (grade 1 and 2) arthritis. Macroscopically, the involved joints were enlarged, swollen and stiff and deformity and ankylosis of affected joints were noticed in severe cases. Deep healing ulcers in planter of the paw were diagnosed in 14 rats. Histopathologic examinations revealed infiltration of mononuclear inflammatory cells and papillary hyperplasia of synovial membrane in mild and moderately affected animals (grade 1 and 2) and erosion of articular cartilage with formation of fibrovascular pannus on its surface in synovial membrane in severe cases (grade 3). In very severe cases (grade 4), complete destruction of articular cartilage and access of granulation tissue to subchondral bone were noticed. Pathologic lesions of the involved joints were very similar to those ...