The effects of intermediate-term immobilization on the medial collateral ligament (MCL) of the rabbit knee are profound. Immobilization causes atrophy with reduction of collagen mass and increased collagen degradation in th immobilized MCL. The matrix composition, as reflected by collagen turnover, changed dramatically as the period of immobilization was increased from nine to 12 weeks. Collagen degradation during these three additional weeks increases, as demonstrated by the fact that at 12 weeks, collagen degradation is 28% higher than that in controls, compared with 14% at nine weeks. Degradation, balanced by new collagen synthesis, produced very minor net decrease in total collagen mass (-2%) after nine weeks, but a large decrease in total collagen mass (-27%) is observed at the end of 12 weeks. Exponential degradation has occurred as immobilization has continued, and ligament regenerative capacity apparently has failed.