The knees of young male rats were immobilized with wire loops passed around the femur and tibia. The knees were examined histologically after sacrificing the animals at intervals from twenty-seven to 123 days. It was found that some flattening of the joint surface occurred so that the small areas of initial contact were enlarged. There was some increase in thickness of the articular cartilage at the point of contact through which an increased amount of weight per unit area was being transmitted. There was no degeneration at the point of contact, but degeneration of the articular cartilage occurred in the non-contact areas. Invasion of the periphery of the articular cartilage by synovial membrane was found.