We examined whether capsaicin evokes the release of glutamate from unmyelinated primary afferent fibers, using a fluorometric on-line monitoring system, in which immobilized glutamate dehydrogenase column was connected to an in vitro superfusion system. In this system, an application of capsaicin (1 microM) resulted in an increased outflow of glutamate from dorsal horn slices in a tetrodotoxin-resistant manner, while capsaicin (3 microM) was without effect on ventral horn slices. This effect of capsaicin on the dorsal horn slices was in a concentration-dependent manner at 0.1-3 microM and the effect at 10 microM was not larger than that at 3 microM. The competitive capsaicin antagonist capsazepine significantly reduced the release of glutamate evoked by 1 microM capsaicin. Twice exposures of dorsal horn slices to capsaicin led a significant decrease in the release of glutamate evoked by the second capsaicin stimulation, indicating desensitization to capsaicin. Thus, using on-line monitoring system, we actually demonstrated that capsaicin induced the release of glutamate from the dorsal horn, probably from capsaicin-sensitive primary afferent terminals.