Although numerous procedures have been described for the reconstruction of the anterior and posterior cruciate ligaments, there has been little evaluation of the viability and strength of these substitutes. Using microangiographic, histological, and biomechanical techniques, we studied the vascularity and tensile strength of the medial one-third of the patellar tendon at intervals after it had been inserted as a substitute for either the anterior or the posterior cruciate ligament in twenty-nine young adult Rhesus monkeys. For the anterior cruciate reconstruction (nineteen knees), we used and medial one-third of the patellar tendon elongated by a portion of the patella. For the posterior cruciate reconstruction (ten knees), we used the medial one-third of the patellar tendon lengthened by attached portions of the patella and tibia as a free graft. Both the anterior and the posterior cruciate ligament substitutes were revascularized at eight weeks, and at one year they had approximately 80 per cent of the tensile strength that they had prior to transfer.