To simulate the forces from hopping, the right foot of adult rabbits was subjected to 1½ the animal's body weight 40 times a minute for 20–40 minutes per day. During these brief periods of repetitive impulsive loading the legs were held in short-leg splints to eliminate the natural shock-absorbing mechanism associated with ankle dorsiflexion and calve muscle stretching. Under these conditions subchondral bone stiffening occurred and was assooiated with the earliest metabolic changes of cartilage damage. When bone stiffening returned to normal the effect on the cartilage did not completely disappear, although these effects diminished. The results suggested that subchondral bone stiffening accompanies the earliest metabolic changes in osteoarthrosic chondrocytes and suggests that trabecular microfracture may occur very early in this sequence of events.