Several amino acids including aspartate, glutamate and glycine and the monoamine serotonin were retrieved from the extracellular space of the dorsal horn of the lumbar spinal cord in the alpha-chloralose anesthetized cat in vivo using a transverse microdialysis probe. Neurotransmitter concentrations were determined using high pressure liquid chromatography in combination with fluorescence (amino acid) or electrochemical (serotonin) detection. Intradermal injection of 3% capsaicin into the hindleg either ipsilateral or contralateral to the dialysis probe was used to evoke release. Extracellular concentrations of aspartate, glutamate and serotonin increased significantly following capsaicin injection into the ipsilateral limb. An almost equal increase in serotonin and a less pronounced, but still significant, increase in aspartate accompanied contralateral capsaicin injection. Glutamate concentrations increased in the dialysate during contralateral capsaicin injection in about half of the animals. These data are consistent with the hypothesis that Asp and Glu are both neurotransmitters released from nociceptive primary afferent fibers and/or interneurons activated by these fibers. In addition, Asp is presumed to be released from intrinsic spinal or descending systems following nociceptive stimulation. Bilateral release of 5HT into the dorsal horn most likely results from non-topographic activation of descending endogenous analgesia pathways.