Growth factors, such as transforming growth factor beta (TGF-b), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), insulin-like growth factor (IGF), basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) are critical components of the wound healing process. Little is known, however, about the expression of these growth factors in flexor tendon healing. In this study, we evaluated the canine flexor tendon at 10 and 21 days after laceration and repair, using immunohistochemical analysis to observe the expression of these growth factors, the effect on expression of different timing of initiation of postoperative therapy, and the association of differences in the healing process. The flexor digitorum profundus tendons in fifteen adult mongrel dogs were lacerated at a clinically relevant point where the pulley and the tendon repair would interact during flexion. The tendon was then repaired with a modified Kessler suture of 4-0 looped supramid (S Jackson, Arlington, VA, USA) and a running suture of 6-0 nylon (Ethicon), and the operated limb was immobilized with a combination of splinting and radial neurectomy (to prevent weight bearing). The tendons were divided into four groups, A: day 1 mobilization & day 10 harvest (n=4), B: day 5 mobilization & day 10 harvest (n=3), C: day 5 mobilization & day 21 harvest & no adhesion (n=4), D: day 5 mobilization & day 21 harvest & adhesion (n=4). Immunohistochemical staining was performed for TGF-b, EGF, PDGF-AA, PDGF-BB, IGF, VEGF and bFGF (R&D System, Minneapolis, MN, USA). To evaluate the positive staining, light microscopy was used and images were saved in an image capture database. The positive staining and total area of the repair site were analyzed using Scion imaging software (Scion Corporation), after construction of montages using an image editing program (Adobe Photoshop Elements). TGF-b, EGF, PDGF-AA, PDGF-BB, IGF, bFGF and VEGF were all expressed at 10 and 21 days after injury, but by different cell types and in different locations. The time at which therapy begins does not appear to affect this expression, at least at day 10. The time course of growth factor expression is an important element in wound healing, and a better understanding of where and when such factors are expressed may help in the development of methods to manipulate this expression, accelerate healing, and reduce adhesions.