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The purpose of this study was to determine whether or not a single application of platelet-rich plasma (PRP) in a ruptured tendon alters the expression of IGF-I in the early phase of healing in an animal wound model. We performed an Achilles tendon rupture model on 48 New Zealand white rabbits, by transecting the tendon transversely and then injecting 0.5 mL of PRP into the tendon mass on one side, and injecting saline on the contralateral, control side. Twenty-four animals received PRP (PRP group), and 24 animals served as untreated controls (control group). Six animals (12 limbs) were killed from each group at 1, 2, 3, and 4 weeks postoperatively. After the animals were killed, 6 paraffin sections were made from each Achilles tendon, 3 of which were stained with hematoxylin and eosin and subjected to microscopic examination, and 3 of which were immunostained with an anti-IGF-I primary antibody. Density of brown diaminobenzidine (DAB) staining was evaluated to quantitatively analyze the results. IGF-I was expressed intracellularly in various cell types throughout the entire healing phase. The growth factor was localized in the epitenon and the endotenon, with an overexpression in the epitenon in the PRP group by the fourth week in comparison with the controls. Furthermore, the tendons treated with PRP healed more rapidly. Based on these findings, PRP could be useful to surgeons treating ruptured tendon.