1. Tonic inhibition of sensory spinal neurones is well known to descend from the rostroventral medulla. It is not clear if this inhibition is dynamically activated by peripheral noxious stimuli. 2. Transection of the ipsilateral dorsolateral funiculus (DLF) removed a descending inhibition of multireceptive spinal neurones and disproportionally prolonged the after-discharge component of their response to a noxious cutaneous stimulus. 3. Microinjection of GABA or tetracaine into the medullary nucleus gigantocellularis pars alpha (GiA) similarly prolonged the after-discharge in response to noxious stimuli. 4. Recordings of GiA cells, initially using minimal surgery, revealed that many had low levels of spontaneous activity and responded vigorously to noxious stimuli applied to any part of the body surface. One hour after the surgery necessary to expose the spinal cord, GiA cells had a high firing rate but responded weakly to noxious stimuli. 5. The response of GiA cells to noxious stimuli was abolished by transection of only the DLF contralateral to the stimulus. 6. It is concluded that the inhibition of multireceptive dorsal horn neurones from GiA is dynamically activated by noxious cutaneous stimuli via a projection in the contralateral DLF. Surgical exposure of the spinal cord tonically activates this inhibition and masks the dynamic component.