The effect of nucleus raphe magnum (NRM) stimulation on the abnormal activity of lumbar dorsal horn neurones in rats with a chronic nerve constriction injury was studied. Neurones on the two sides of the spinal cord were extracellularly recorded in anaesthetized and curarized preparations. The neurones on the side ipsilateral to the injury showed a spontaneous firing frequency higher than that of the opposite intact side (25.8 +/- 1.9 spikes s-1 vs 4.5 +/- 1.3 spikes s-1). The hyperactivity was reduced to a mean value of 9.3 +/- 1.3 spikes s-1 by NRM stimulation using parameters able to selectively inhibit the noxiously evoked neuronal responses on the intact side. The results indicate that the hyperactivity is a phenomenon related to chronic pain that may be modulated by the endogenous pain control system.