Experimental osteoarthritis has been studied by various methods such as local compression of the joint, immobilization of the joint, relief of the contact and incision of the meniscus. The instability caused by the ligament injuries of the knee joints of the rabbits induced degenerative changes of the articular cartilages. After excision of the medial collateral ligament, medial meniscus and both cruciate ligament, the articular cartilages were observed serially after 1, 2, 3, 4, 6, 8, 10 and 12 weeks, grossly and microscopically. The results obtained are as follows; 1. Loss of transluscency and luster was observed even in the early group of the first week, and was followed by erosion and fissure on the articular cartilage in the groups of the third and later weeks grossly. 2. Microscopically, cluster formation of the chondrocytes on the articular cartilage was observed in the group of the first week and there was tendency of increase in its numbers and extents in the subsquent groups. Fibrillation and cleft formation was also observed in these groups. 3. Loss of metachromasia was observed in the groups of the second and subsquent weeks, but it was most prominant in the groups of the sixth and eighth weeks. 4. The tendancy of cluster formation was more prominent, compared with those of fibrillation or erosion.