Although many different attempts have been made to reduce adhesions after tendon surgery, this complicated problem continues to be foremost among factors compromising tendon surgery and repair. The results of the finest tendon surgery and repair are frequently compromised by adhesions that restrict motion, decrease function and often lead to permanent deformation. Many substances and materials have been introduced into the area around primary flexor tendon repairs in an effort to prevent or diminish the adhesion formation. Various biochemical agents including antihistamines, anaboilic agents, lathyrogenic drug, betaamino-propionitrile and steroids, have been investigated and material such as nylon, cellophane, polyethylene film and silastic have been used to form pseudosheath. However the results with all of these method have been disappointing. Recently, interest has grown in a mucopolysaccharide found in synovial fluid hyaluronic acid. Synovial fluid normally contains a concentration of 2-3 mg/ml of hyaluronic acid. Preliminary investigations suggest that hyaluronic acid placed between the tendon and its sheath promote tendon healing and decrease adhesion formation. With this in mind, the present study was designed to examine the effect of hyaluronic acid on injured chicken flexor tendon healing and to determine the quantity and quality for adhesions in hyaluronic acid treated chicken compared to control. The stereomicroangiographic technique has enabled us to study the vascular process inside the tendon and surrounding tissues after injury of the tendon. For the study, the deep flexor tendon of the third toe of adult chicken with initial weight of about 2kg was used. Animals were divided into three groups and treated with different levels of hyaluronic acid. After 1,3,7,21,35 and 42 days postoperation, the animals were sacrificed and specimen were prepared. Obtained specimen were examined macroscopically and microscopically. At 1,3,5, and 7 weeks postoperation, microangiographic study were done and examind specimen by stereomicroscope. The results were as follows: 1. Though both the treated and control groups showed adhesion, hyaluronic acid treated tendon showed less adhesion tissue and better sliding properties as compared to the controls. 2. During the first week after operation, microangiographic studies showed failure of filling of all the vessels within the injured tendon. At 3 weeks highly vascular adhesions which extended throughout the injured site were observed. After then, the amount of vascularity decreased. 3. There were no difference microscopically between hyaluronic acid treated and control group at first week. After one week hyaluronic acid treated group showed less granulation tissue and less formation of collagen fibers. 4. There were no difference of healing process between hyaluronic acid treated and control group. From this results, it is suggested that hyaluronic acid is effective in reducing the adhesions after the tendon surgery.