Chemical synovectomy with osmic acid appears to be one of the means of decreasing inflammation, effusion and pain in rheumatoid arthritis. Authors injected 1% osmic acid to rabbit knees, then the changes of the articular cartilage and synovium were observed. 1. Grossly, the color of the cartilage was dark brown and progressive degenerative changes were noted. Also the color of the synovium turned dark brown and the synovium became thicker. 2. Radiologically, the degenerative changes appeared at the 16th week. 3. Light microscopically, the degenerative changes of the cartilage were progressed. Most synovium were necrotic at the early phase and with time, the fibrosis was more progressed. At the 2nd week, there were layers which were composed of the synovial like cells and the regeneration process was almost completed at the 16th week. 4. Transmission electron microscopically, there was destruction of the chondrocytes of superficial layer at the 1st day, and the intracellular organells of the chondrocytes dispersed extracellulary at the 16th week. Osmic acid particles were still obsereved in the macrophage in the synovium at the 16th week. 5. Scanning electron microscopically, Some minute craters appeared in the surface of the cartilage at the 1st day and were transformed to cleft and destructive changes at the 16th week. Above findings suggested that intraarticular osmic acid injection might be used for temporary synovectomy effect, but destruction of articular cartilage and residual hypertrophy of synovial and periarticular tissue are the problems. Intraarrticular osmic acid injection for synovectomy effect should be used with careful consideration.