The knee joints of live guinea pigs, subjected to repeated longitudinal impaction, developed obvious cartilage degeneration over a 3 week period. In vitro tests had previously shown that articular cartilage is particularly susceptible to injury from impact loading and that subchondral bone acts as a cushion to protect the overlying cartilage from damage during such loading. Associated with, and slightly preceding the earliest cartilage changes, as judged histochemically, was a stiffening of the underlying subchondral bone. The bone stiffness measurements returned to within the normal range as the cartilage degeneration progressed.