Chronic constriction injury of the sciatic nerve and lumbar L5 and L6 spinal nerve ligation provide animal models for pain syndromes accompanying peripheral nerve injury and disease. In the present study, we evaluated changes in brain-derived neurotrophic factor (BDNF) immunoreactivity in the rat L4 and L5 dorsal root ganglia (DRG) and areas where afferents from the DRG terminates (the L4/5 spinal cord and gracile nuclei) in these experimental models of neuropathic pain. Chronic constriction injury induced significant increase in the percentage of small, medium and large BDNF-immunoreactive neurons in the ipsilateral L4 and L5 DRG. Following spinal nerve ligation, the percentage of large BDNF-immunoreactive neurons increased significantly, and that of small BDNF-immunoreactive neurons decreased markedly in the ipsilateral L5 DRG, while that of BDNF-immunoreactive L4 DRG neurons of all sizes showed marked increase. Both chronic constriction injury and spinal nerve ligation induced significant increase in the number of BDNF-immunoreactive axonal fibers in the superficial and deeper laminae of the L4/5 dorsal horn and the gracile nuclei on the ipsilateral side. Considering that BDNF may modulate nociceptive sensory inputs and that injection of antiserum to BDNF significantly reduces the sympathetic sprouting in the DRG and allodynic response following sciatic nerve injury, our results also may suggest that endogenous BDNF plays an important role in the induction of neuropathic pain after chronic constriction injury and spinal nerve ligation. In addition, the increase of BDNF in L4 DRG may contribute to evoked pain which is known to be mediated by input from intact afferent from L4 DRG following L5 and L6 spinal nerve ligation.