Mechanical properties of the stress-shielded patellar tendon were studied in the rabbit knee. Stress shielding was accomplished by stretching a stainless-steel wire installed between the patella and tibial tubercle and thus, releasing the tension in the patellar tendon completely. Tensile tests were carried out on the specimens obtained from the patellar tendons which were exposed to the stress shielding for 1 to 6 weeks. The stress shielding changed the mechanical properties of the patellar tendon significantly: it decreased the tangent modulus and tensile strength to 9 percent of the control values after 3 weeks. There was a 131 percent increase in the cross-sectional area and a 15 percent decrease in the tendinous length. Remarkable changes were also observed in the structural properties: for example, the maximum load of the bone-tendon complex decreased to 20 percent of the control value after 3 weeks. Histological studies showed that the stress shielding increased the number of fibroblasts and decreased the longitudinally aligned collagen bundles. These results imply that if no stress is applied to the autograft in the case of augmentative reconstruction of the knee ligament, the graft strength decreases remarkably.