Several lines of evidence have suggested the participation of the caudal medulla, including the subnucleus reticularis dorsalis (SRD), in the supraspinally mediated diffuse noxious inhibitory controls (DNICs). To test this hypothesis directly, DNICs acting on spinal convergent neurones were compared in sham-operated rats and rats with unilateral quinolinic acid-induced lesions of the SRD. Inhibitions produced by heterotopic noxious stimuli (i.e. DNIC), of the C-fibre-evoked responses of the convergent neurones were significantly reduced in the lesioned animals. This depression of DNIC, by 40-45%, was similar no matter where the conditioning stimuli were applied and was not significantly different for neurones recorded ipsilaterally to the lesion from those recorded contralaterally. These results suggest that the SRD is one of the supraspinal relay(s) in the circuitry underlying DNIC in the rat.