The effects of capsaicin systemically administered in adult rats, with the major focus on the expression of brain-derived neurotropic factor (BDNF) and its mRNA in the dorsal root ganglion (DRG) and spinal cord, has been investigated by means of immunohistochemistry and reverse transcriptase-polymerase chain reactions. The percentage of BDNF-immunoreactive neurons in the L5 DRG was found to increase significantly 1 day after capsaicin injection. Subsequently, it decreased slowly returning to near normal levels 1 week later. Four weeks post-injection, a significant reduction to below normal levels was observed. The temporal pattern of BDNF mRNA expression in the DRG was similar to BDNF-immunoreactivity. In the spinal cord, 1 and 3 days post-injection, no changes in the expression of the BDNF-immunoreactive axonal fibers was noted. However, the expression had decreased significantly after 1 and 4 weeks. The mechanism by which capsaicin induces changes in expression of BDNF in DRG neurons and the functional significance of the rapid increase in BDNF levels in the DRG is discussed briefly.