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SUMMARY1. In guinea-pigs hypothalamic single units were extracellularly tested for their response to thermal stimulation of the skin and to electrical stimulation of two different pontine areas, the nucleus raph6 magnus and the dorsomedial reticular formation. Furthermore, thermoregulatory control actions were measured during the stimulations.2. Electrical stimulation of those reticular formation areas containing noradrenaline cells caused an increase of oxygen uptake, electrical muscle activity and body temperature, while stimulation of the nucleus raphe magnus, known to contain serotonin cells, brought about inhibition or had no effect.3. The recorded units could be subdivided into three groups.Cell type a. Neurones on the boundary of preoptic and anterior hypothalamic regions which increased their firing rate when the skin was cooled and decreased it when the nucleus raphe magnus was stimulated. Cell type b. Neurones in the anterior hypothalamus which did not respond to brain-stem stimulation. Cell type c. More posterior neurones which increased their firing rate when the skin was warmed or when the nucleus raph6 magnus was stimulated and decreased their firing rate when the reticular formation was stimulated.4. Cell type a seems to represent interneurones which are connected to the ascending serotonergic thermoregulatory pathway. As for cell type c, it is inferred that it could represent interneurones which control the threshold for shivering and non-shivering thermogenesis.

SUMMARY1. The responses of fifty-five single units to changes in skin temperature were recorded in twenty-three guinea-pigs anaesthetized with urethane. Skin-temperature changes were induced by changing the temperature of the water-perfused support plate of the stereotaxic apparatus and that of the double-walled Perspex jacket that was put on the support plate.2. Thirty-three units were stereotaxically and histologically verified as being within a circumscribed area of the pontine dorsomedial reticular formation (subcoeruleus region). Twenty-one units were located in the surrounding areas, and one unit in the nucleus raphe magnus region.3. Twenty-seven of thirty-three recorded subcoeruleus units were specifically excited by cooling of the abdominal or leg skin, whereas only five units were non-thermoresponsive and one unit was warm-responsive. The cold-responsive units had peak activity at skin temperatures between 22 and 29 IC, in accordance with the maximum activity in cutaneous cold-receptors. 4. A markedly different distribution of units was found in the surrounding areas. Only four units were cold-responsive. Thirteen units were non-thermoresponsive, and four units were warm-responsive.5. The cold-responsive subcoeruleus units were situated in regions which are known to contain accumulations ofnoradrenergic cell bodies, and to project to hypothalamic neurones. Electrical stimulation of these regions is known to cause excitatory metabolic responses in unanaesthetized guinea-pigs. It is concluded that part of the cutaneous cold-afferents projects to hypothalamic thermointegrative neurones via noradrenergic pathways that ascend from these subcoeruleus regions.