The aim of this study is to elucidate whether the excitatory amino acid glutamate is released from capsaicin-sensitive primary afferent fibers, and to compare the releasing effect of capsaicin on glutamate with that on substance P. The release of glutamate was measured using a fluorometric on-line continuous monitoring system, in which the immobilized glutamate dehydrogenase column was connected to an in vitro superfusion system. In the presence of 0.3 microM tetrodotoxin, 2-min application of capsaicin produced an increased outflow of glutamate, as well as an increase in the release of immunoreactive substance P from dorsal horn slices of the rat. The release of glutamate was concentration-dependently increased by capsaicin at concentrations in the range of 0.1-3 microM, and the release evoked by 10 microM capsaicin was not higher than that evoked by 3 microM. On the other hand, capsaicin at concentrations of 1-10 microM produced a concentration-dependent increase in the release of immunoreactive substance P, without effect at 0.1 microM. The amount of glutamate release evoked by 3 microM capsaicin was about 42.8 pmol.mg-1 protein, and 290 times that of immunoreactive substance P. The release of glutamate by 3 microM capsaicin was suppressed by the depletion of calcium from the superfusate. Capsaicin at 3 microM failed to increase the release of glutamate from the dorsal horn slices of the rats made an L4-L6 dorsal rhizotomy. These results suggest that capsaicin evoked the release of glutamate from primary afferent fibers in the dorsal horn and that glutamate may play an important role in pain transmission between primary afferent fibers and dorsal horn neurons.