Opinions differ on the effects of running on joints. The progression of osteoarthritis and selection makes it difficult to evaluate the effects of increased motion load on joints. The effects of treadmill running on experimentally produced unilateral osteoarthritis of the knee was estimated in a matched-pair study with rabbits. One rabbit from each of 10 pairs was randomly chosen to run at a near maximal level over a 14-week period. The stage of osteoarthritis before and after the running period was estimated from mobility measurements and radiographs, and after running from histological sections or macroscopic appearance. There were no clear differences between the exercised and control rabbits; in both groups there was a slight progression in the degenerative changes of the osteoarthritic and contralateral joints. Near maximal running had no negative effects on osteoarthritic or contralateral rabbit knees in this study.