The effects of altered levels of activity on the biomechanical properties of a ligament were investigated. After eight weeks of immobilization, anterior cruciate bone-ligament-bone preparations of wild primates tested in tension showed significant decreases in maximum failure load and energy absorbed to failure (39 per cent and 32 per cent), and an increase in ligament extensibility (compliance). These changes indicated an altered functional capacity of the ligament unit. The long-term effects of disuse were shown in a second group. After twenty weeks of resumed activity following immobilization, there was only partial recovery in ligament strength, although ligament compliance had nearly returned to normal. In a third group, a voluntary isotonic exercise of one lower limb performed by the primate during immobilization did not prevent disuse-induced changes in ligament failure properties. Ligament properties after immobilization were influenced by: (1) changes in the ligament force-elongation relationship; (2) histological changes at the bone-ligament junction; (3) altered mechanisms of specimen failure; and (4) presumed changes in the glycosaminoglycan and collagen fiber relationship.