The in vitro rat spinal dorsal horn slice--dorsal root ganglion (DRG) preparation and high performance liquid chromatography (HPLC) were used to examine the release of nine endogenous amino acids (glutamate, aspartate, glutamine, asparagine, glycine, [gamma]-aminobutyric acid, serine, threonine and alanine) from the spinal dorsal horn, in response to electrical or chemical activation of different categories of primary afferent fibers (PAF). In addition, a possibility of modulation of the basal and the dorsal root stimulation-evoked release of the nine endogenous amino acids by substance P (SP), neurokinin A (NKA), calcitonin gene-related peptide (CGRP), opioid peptides, and by the activation of GABA[subscript] B receptors, has been examined;Selective activation of the low-threshold (A[beta]) PAF resulted in a two- to three-fold increase in the outflow of endogenous aspartate (ASP) and glutamate (GLU) into the spinal superfusate. Activation of both the low- and the high-threshold (A + C) PAF elicited a greater increase in the outflow of GLU and ASP and a marked increase in ASP/GLU outflow ratio. Superfusion of the DRG with capsaicin or resiniferatoxin produced a prolonged two- to three-fold increase in the outflow of both GLU and ASP into the superfusate;Low concentrations (0.2 [mu]M) of SP caused a selective increase in the basal outflow of GLU, whereas higher concentrations (1-5 [mu]M), in addition to GLU, evoked an increase in ASP. The enhancement of the basal outflow of GLU by SP persisted in the absence of external Ca[superscript]2+ but it was blocked by (D-Arg[superscript]1, D-Pro[superscript]2, D-Trp[superscript]7,9, Leu[superscript]11)-SP, a claimed antagonist of synthetic SP. CGRP (10[superscript]-7) caused a significant increase in the basal outflow of GLU and ASP and a decrease in asparagine. This effect was Ca[superscript]2+-independent. Neonatal capsaicin treatment prevented the SP-induced release of GLU whereas the effect of CGRP was not significantly modified. SP and CGRP enhanced, whereas DAGO and baclofen (agonists at [mu]-opioid and GABA[subscript] B receptors, respectively), reduced the evoked release of GLU and ASP;Our results have provided further evidence in support of contention that Glu and Asp act as excitatory neurotransmitters of the PAF. In addition, the results indicate that tachykinins and CGRP may enhance the basal and evoked release of endogenous GLU and ASP, whereas opioid peptides acting at [mu]-receptors, and the activation of GABA[subscript] B receptors, suppress the evoked release of the two amino acids in the spinal dorsal horn.