The role of revascularization in the nutritional support of repair of the flexor tendons is not completely understood. To explore the extent to which intrasynovial flexor tendons revascularize after transection and suture, a vascular injection study was carried out in a canine model. The tendons to the second and fifth digits of the forepaw in twelve adult mongrel dogs were transected and repaired. There were twenty-four experimental tendons and twenty-four normal tendons. The limb was placed in a polyurethane shoulder-spica cast, and the paw was treated with immediate protected passive mobilization. At three, seven, ten, seventeen, and twenty-eight days, the animals were killed and the major arteries supplying both the paw that had been operated on (left) and the contralateral normal paw (right) were injected with 200 milliliters of India ink. Segments of repaired and normal tendons were then clarified by a modified Spalteholz technique. The normal tendons demonstrated a well developed mesotenon that provided vascularization of the proximal portion of the flexor digitorum profundus tendon. A consistent three-cubic-millimeter avascular intrasynovial portion of tendon was noted. Distally, vessels arose from the vinculum breve, supplying the terminal twenty millimeters of tendon substance. In the experimental tendons, longitudinal and transverse clarified sections showed consistent revascularization of the site of repair by proximal vessels in the absence of ingrowth of peripheral adhesions. Vessels in the epitenon progressively extended for a distance of ten millimeters, through normally avascular regions, to reach the site of repair by the seventeenth postoperative day. Intratendinous vessels about the site of repair consistently originated from surface vessels, rather than from extensions of pre-existing intratendinous vessels. New vessels penetrated all areas, including the normally avascular volar segments of tendon, irrespective of previous topical zones of avascularity. Proximal vascular plexi were characterized by large tortuous vessels with frequent circuitous branches. More distal vessels had a longitudinally oriented, feathery appearance.