Peripheral nerve injury results in plastic changes in the dorsal ganglia (DRG) and spinal cord, and is often complicated with neuropathic pain. The mechanisms underlying theses changes are not known, but these changes seem to be most likely related to the neurotrophic factors. This study investigated the effects of mechanical peripheral nerve injury on expression of brain-derived neurotrophic factor(BDNF) in the DRG and spinal cord in rats. 1) Bennett model and Chung model groups showed significantly increased percentage of small, medium and large BDNF-immunoreactive neurons in the ipsilateral L₄ DRG compared with those in the contralateral side at 1 and 2 weeks of the injury. 2) In the ipsilateral L5 DRG of the Chung model, percentage of medium and large BDNF-immunoreactive neurons increased significantly at 1 week, whereas that of large BDNF-immunoreactive neurons increased, but that of small and medium BDNF-immunoreactive neurons decreased at 2 week when compared with those in the contralateral side. The intensity of immunoreactivity of each neuron was lower in the ipsilateral than in the contralateral DRG. 3) In the spinal cord, the Bennett and Chung model groups showed a markedly increased BDNF-immunoreactivity in axonal fibers of both superficial and deeper laminae. The present study demonstrates that peripheral nerve injury in neuropathic models altered the BDNF expression in the DRG and spinal cord. This may suggest important roles of BDNF in sensory abnormalities after nerve injury and in protecting the large-sized neurons in the damaged DRG.