The two-stage silicone-rod technique for flexor-tendon replacement was performed in the forepaws of twenty-four mongrel dogs and in the hands of four chimpanzees. The graft and sheath complex was studied at intervals from two weeks to one year after the second-stage replacement, by injection of the vascular tree with silicone compound and by histological techniques. Revascularization was well developed by four to six weeks after replacement. At four to six months, parietal and visceral sheaths, connected by an elastic areolar membrane, were well differentiated and highly vascularized. The reticular mesotenon served as a non-restricting, gliding bridge conveying vessels to and from the tendon. Many longitudinal and transverse connecting vessels were present in the closely adherent, thin, transparent epitenon (visceral layer). These vessels, as well as intratendinous vessels, were present throughout the entire length of the tendon, which in the chimpanzee is as long as a human flexor tendon.