FK506 is an immunosuppressant drug that has been shown experimentally to stimulate nerve growth and speed functional recovery, when administered immediately after peripheral nerve injury. However, the clinical scenario of a peripheral nerve injury is often associated with either a delayed diagnosis or reconstruction. The purpose of this study was to determine the efficacy of FK506 on neuroregeneration with delayed administration. Thirty-two Lewis rats underwent tibial nerve transection with immediate repair. Animals were left untreated, or were treated with daily injections of FK506 (2 mg/kg), started on the day of surgery, postoperative day 3, or postoperative day 5. Animals underwent walking track analysis to assess functional nerve recovery. Nerves were harvested for histomorphometric analysis on postoperative days 21, 28, and 42. Histomorphometry demonstrated that all treatment groups, regardless of the time of drug initiation, demonstrated evidence of enhanced neuroregeneration, compared to the untreated group. Histomorphometric data from groups harvested on day 21 demonstrated a statistically significant improvement in neuroregeneration in the immediate and 3-day delay groups. Therefore, the beneficial effects of FK506 on neuroregeneration are not restricted to immediate administration, but these effects significantly diminish when FK506 is administered 3 days after nerve injury.