1. Superfusates taken from spinal cords of cats anaesthetized with chloralose and urethane were assayed for endogenous serotonin and noradrenaline by high-pressure liquid chromatography with electrochemical detection. 2. Stimulating the dorsolateral funiculus, caudal to a spinal transection, enhanced in a frequency-dependent manner the levels of monoamines in the spinal superfusate. 3. Tyramine added to the superfusate enhanced the release of noradrenaline and serotonin. 4. In cats with intact neuraxes, stimulation of the sciatic nerve at high, but not low intensities produced a 2- to 3-fold increase in the levels of monoamines in the spinal superfusate. This evoked monoamine efflux was attenuated by cold block of the cervical spinal cord. 5. Stimulation of the infraorbital branch of the trigeminal nerve evoked the release of noradrenaline and serotonin from the lumbar cord in animals with intact neuraxes. Cold block of the cervical cord blocked trigeminal-evoked release of lumbar serotonin and noradrenaline. 6. That the monoamine efflux was not due to elevations in blood pressure was indicated by the failure of vasoxyl, an alpha-agonist producing hypertension, to evoke any changes in spinal monoamine levels. 7. The monoamine release was not dependent upon either an opiate-sensitive link or upon the activation of the sympathetic ganglia, because systemic administration of naloxone (an opiate antagonist) and chlorisondamine (a ganglionic blocking agent) failed to antagonize the evoked release of amines. 8. These results suggest the existence of a spinopetal monoamine system which is activated by peripheral stimuli. The modulatory influence associated with increasing monoamine tone in the spinal cord clearly indicated that somatic stimuli may activate a descending monoamine pathway which serves to modulate the magnitude of the ascending sensory message.