Tacrolimus (FK506) is an immunosuppressive medication that promotes organ allograft survival. It has also been shown to enhance nerve regeneration and muscle reinnervation in animals but these properties have not previously been studied in patients. Moreover, currently there is no method in clinical use to speed the rate of recovery after nerve injury. The objective of this study is to explore the ability of tacrolimus to benefit the treatment of patients with peripheral nerve injury. To minimize the morbidity of tacrolimus therapy, its phase-specific effects on nerve regeneration and muscle reinnervation will be defined in the murine model to permit further limitation of the duration of therapy. The investigators hypothesize that treatment with tacrolimus after autogenous peripheral nerve reconstruction will accelerate nerve regeneration, reduce the period of denervation and improve muscle reinnervation and recovery in patients with peripheral nerve injury.