After a unilateral hippocampal fornix-fimbria transection in adult rats, either autologous peripheral nerve or fetal hippocampus was implanted into the transection site. After 2 to 4 weeks, 2 to 3 months, and 6 to 8 months fimbria fiber regeneration was analyzed by acetylcholinesterase (AChE) histochemistry and retrograde transport of horseradish peroxidase after injection into the denervated host hippocampus. Prominent innervation of both types of implant by central AChE-staining axons occurred by 2 to 3 weeks postimplantation and was sustained to at least 8 months. Reinnervation of the adjacent host hippocampal terminal zone was also apparent, but was sparse compared with innervation of implants.