Possible correlation of release of endogenous glutamate (Glu) and aspartate (Asp) with stimulation parameters used to activate primary sensory neurons was examined using the rat spinal cord slice--dorsal root ganglion preparation and high performance liquid chromatography with fluorimetric detection. Selective activation of the low-threshold (A beta) primary afferent fibers resulted in a two-fold increase in the rate of basal outflow of Asp and a smaller increase in the outflow of Glu from the rat spinal dorsal horn slices into the superfusing medium. The activation of both the low (A beta)- and the high-threshold (A delta + C) primary afferents elicited also a significant increase in the outflow of Asp and Glu relative to control. Glu and Asp are released in significant amounts following superfusion of the dorsal root ganglia with capsaicin or resiniferatoxin. DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol-enkephalin), an agonist at mu-opioid receptors, attenuated the high-intensity stimulation-evoked outflow of Asp and Glu in a naloxone-sensitive manner. Our results have provided further evidence in support of the contention that Glu and Asp act as excitatory synaptic transmitters in the spinal dorsal horn. A role for mu-opioid receptors in modulation of spinal processing of somatosensory information is indicated.