Ten nociceptive neurones located deep (greater than 640 microns) in the dorsal horn of the intact halothane anaesthetized rat were activated by transcutaneous electrical stimulation of their receptive fields producing short latency A beta and C fibre evoked activity. Repetitive C fibre stimulation (16 stimuli at 0.5 Hz) produced "wind up", an increase in the evoked activity with suceeding stimuli and consequently post discharges of the neurones. Direct application of aminophosphonovaleric acid (AP5) onto the cord surface (250 or 500 micrograms) produced minor changes in A and C fibre inputs onto the cells but reliably reduced "wind up" and the post discharges of the neurones. The results suggest an involvement of the NMDA receptor in this potentiation of dorsal horn nociceptive neurones.