Although electrical stimulation of supraspinal structures and local large fibres is known to inhibit the responses of nociceptive neurones, comparable studies on thermoreceptive cells have not been made. We have studied the effects of nucleus raphe magnus (NRM) and segmental stimulation on cold and warm responsive neurones in trigeminal nucleus caudalis of both the rat and cat. All 48 neurones (46 cold and 2 warm) tested in the cat and 24 cold neurones in the rat were unaffected by the NRM at a variety of stimulation parameters. However, segmental stimulation inhibited 4/20 neurones in the cat and 11/26 cells in the rat. The results show the selectivity of the inhibition following NRM stimulation.