Effects of insulin-like growth factor I (IGF I) on peripheral nerve regeneration was studied using allografts in a rat sciatic nerve model. Thirty male white Wistar rats were divided into 3 experimental groups (n = 10) randomly: normal control group (NC), allograft group (ALLO), and IGF I–treated group (ALLO/IGF). In the NC group, the left sciatic nerve was exposed through a gluteal muscle incision and, after homeostasis, the muscle was sutured. In the ALLO group, the left sciatic nerve was exposed through a gluteal muscle incision and transected proximal to the tibioperoneal bifurcation where a 10-mm segment was excised. The same procedure was performed in the ALLO/IGF group. The harvested nerves of the rats of the ALLO group were served as allograft for the ALLO/IGF group and vice versa. The NC and ALLO groups received 10 μL of sterile phosphate buffered saline intraperitoneally once a day for 1 week, and the ALLO/IGF group received 10 μL of IGF I (100 ng/kg per day) intraperitoneally once a day for 1 week. Behavioral testing, sciatic nerve functional study and the gastrocnemius muscle mass showed earlier regeneration of axons in the ALLO/IGF group than in the ALLO group (P < 0.05). Administration of IGF I could accelerate functional recovery after nerve allografting in the sciatic nerve and may have clinical implications for the surgical management of patients after facial nerve transection.