The location and properties of the cutaneous receptive fields responsible for detecting the flexor withdrawal reflex in the posterior head of biceps femoris (pBF) and semitendinosus (ST) components of the hamstring muscle have been examined in unanaesthetized decerebrate rats, spinalized at T10-T11. Single alpha-motoneurone efferents were recorded from the nerve to pBF and the principal head of ST and their responses to ipsi- and contralateral hindlimb skin stimulation investigated. The efferents to both muscles characteristically had a low or absent background discharge and they all had mechanoreceptive fields on the ipsilateral foot. The mechanical threshold of these fields was high with no response to light touch or brush. Fifty-four percent of these units also had a smaller and weaker contralateral mechanoreceptive field. The only apparent difference between ST and pBF efferents was that more ST efferents had contralateral fields than pBF units. Noxious, hot and cold thermal stimuli applied to the ipsilateral foot activated 56% of the efferents. Mustard oil, a chemical irritant, produced a long-lasting flexor response when applied to the ipsilateral foot. The responses of these efferents to stimulation of A beta, A delta and C cutaneous afferents in the sural nerve were also studied. Short latency reflexes were elicited in all efferents by A beta inputs, longer latency reflexes were elicited in 64% by A delta inputs and very long latency responses with long afterdischarges were found in 73% of the units to C inputs. Retrograde labelling of the hamstring motoneurones with WGA-HRP indicated that they lay in ventrolateral lamina IX extending from the caudal portion of the third lumbar segment to the junction of the 5th and 6th lumbar segments. Transganglionic labelling of small diameter primary afferent terminals in the dorsal horn of cutaneous nerves innervating the foot revealed that the longitudinal distribution corresponded closely with that of the hamstring motor nucleus. The flex-or reflex in the spinal rat provides a useful model therefore, for studying how the input in nociceptive afferents is processed and transformed within the spinal cord, to produce appropriate outputs.