Fifty-four young rabbits were divided into five groups and each group was tested for 2, 4, 6, 8 and 12 weeks respectively. The right front leg of each rabbit was struck repeatedly with an automatically controlled striking unit. The striking force was 3.5 kg and the intensity was 90 times/min, 60 mins/day with a 10-minute rest in between. Pathological changes as observed by a light microscope were mainly hyperplasia, degeneration atrophy, focal necrosis, clefts and microfracture. These changes were more often seen in the germinal zone, proliferation zone, groove of ossification and perichondral ring. The change in the middle portion was more serious than that in the peripheral portion. Pathomechanism was also discussed in the paper.