The mechanics of pes anserinus function before and after Slocum-Larson31 transplantation were determined by dissection and roentgenography, by mechanical tests on limbs of cadavera, and with the use of an analytical model. The three muscles forming the pes anserinus have a marked functional dependence on the position of the knee joint as regards flexion and external rotation. The muscles primarily are knee flexors but secondarily provide a significant contribution to rotatory stability. Transplantation yields a significant increase in rotatory effectiveness.