The depth of wound level is as important as the level of the tendon injury itself. And the timing of the operative procedure is an important factor. As a rule, tendons injured outside of the flexor sheath yield much better results than those injured within the sheath. The nutritional supply of the flexor tendons is not completely understood. Many elaborate studies have outilned the vascular anatomy of these tendons, and not all authors are in agreement. It is now clear that synovial fluid within the sheath supplies nutrition to the tendon much as synovial fluid in a joint supports cartilage. With this in mind, the present study was designed to determine the effect of exposing tendons for varying periods of time on the viability of the tendon and sequential morphological changes. The results are as follows: 1. Twelve hours after tendon sheath removal, collagen fibrillar dissociation and irregular surface of the tendon sheath were noted on the chicken flexor tendons by electronmicroscopy. 2. Superficial tenocyte necrosis was created after 24 hours of tendon exposure. 3. At 3 days, inflammatory cell infiltration and thickening of the outer synovial layer were noted. After 7 days, fibrosis of the degenerated tendon started from the exposed surface. 4. The fibrous connective tissue and new blood vessel infiltration into the tendon were progressed after 7 days. From these morphological results, any interference with the synovial environment leads to a regressive change of the flexor tendons immediately. It is suggested that delayed primary wound covering procedure within three days an after exposed tendon injury is ideal, and it should by done at least 7 days after tendon exposure.