We sought to examine the likelihood that somatic input into the central nervous system might serve to release serotonin and noradrenaline into the cat spinal superfusate. In these experiments, the lumbar spinal cord of the chloralose-urethanized cat was superfused with a concentric polyethylene catheter. Resting levels of serotonin and noradrenaline were on the order of 0.5 and 0.3 ng/ml, respectively. Following bilateral stimulation of the sciatic nerve at intensities which produced no change in pupil size or blood pressure, there was no change in the resting levels of serotonin and noradrenaline. In contrast, stimulation of the sciatic nerve at intensities which evoked elevations in pupil size and blood pressure, an approximate 3-fold increase in the levels of either monoamine was noted during the interval of stimulation. In other experiments, elevations of blood pressure by intravenous infusion of vasoxyl failed to produce significant changes in the levels of serotonin and noradrenaline in the spinal superfusates. These results suggest that the activation of small diameter, peripheral somatic, afferents activates descending monoamine pathways. One consequence of this activation, as predicted by the effects of the intrathecal administration of these monoamines, would be to modulate the processing of nociceptive information.