1. The responses of single neurones in the nuclei raphés magnus, medianus, dorsalis and pontis to changes in skin temperature were recorded in rats anaesthetized with urethane. Skin temperature was altered by means of a water-perfused jacket. 2. Of 210 neurones studied, thirty-five were specifically excited by warming the skin whilst twenty were cold responsive. The greatest proportion of cells responding to skin temperature were in the nucleus raphé magnus, whilst few neurones in the raphé dorsalis and pontis were influenced. 3. The warm units had peak activity at a mean skin temperature of 37.7 degrees C whilst the cold cells had a corresponding maximal rate at 29.0 degreet C. Mechanical and noxious peripheral stimulation, blood pressure changes and temperatures other than that of skin did not affect the neurones. 4. The neurones influenced by skin temperature were histologically verified as being within the raphé system. 5. LSD inhibited all neurones tested, indicating that the cells were serotonergic. 6. The responses to skin temperature were unchanged in rats with midcollicular sections suggesting an ascending thermal system. 7. The results suggest that any involvement of 5-HT in central thermo-regulation is in terms of an afferent thermal pathway mediated by serotonergic raphé neurones.