There is now considerable evidence that neurons of the ventromedial medulla, including the serotonergic neurons of the raphe nuclei 18, control the excitability of spinal neurons which transmit information about noxious stimuli 6, 8, 16. A direct pathway from this region, which descends in the dorsolateral funiculus of the spinal cord and terminates in dorsal horn laminae I, II and V has recently been demonstrated 1. Physiological studies have shown that this pathway mediates a powerful inhibition of neurons in these laminae which respond maximally to high intensity cutaneous inputs 9. Furthermore, morphine and stimulus-produced analgesia 12 are reduced or abolished after electrolytic or chemical lesions of the ventromedial medulla 15, 19 or of the dorsolateral funiculus z, 10.