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National Institute for Medical Research, Mill Hill, London, NW7 1AA

1. Single unit activity has been recorded in the thalamic, hypothalamic and raphe magnus nuclei of rats anaesthetized with Urethane.

2. Neurones were sought which responded to changes in scrotal skin temperature applied with a water-perfused brass thermode. All sixty-nine neurones in the thalamus and hypothalamus responded with abrupt changes in activity as the scrotum was warmed (`switching response'). The majority responded with an increase in activity from minimal to maximal firing rate as the scrotum was warmed over a range of less than 0·5 °C; in about 20% of the neurones the converse was observed.

3. To determine whether the switching response of the thalamic and hypothalamic neurones depended upon a cortico-thalamic feed-back loop, the cortical surface was cooled to 18-20 °C to reversibly abolish cortical post-synaptic activity.

4. Cortical cooling abolished the positive switching response of nearly all (15/19) ventrobasal thalamic neurones to scrotal warming. All eight ventrobasal thalamic neurones with negative switching responses, and all twenty-two scrotal temperature-responsive neurones in other thalamic and hypothalamic nuclei were unaffected.

5. Twenty recordings were also made from scrotal temperature-responsive neurones in the nucleus raphe magnus. All possessed switching responses similar to those observed in the thalamus and hypothalamus.

6. None of the scrotal temperature-responsive neurones in the nucleus raphe magnus was affected by cortical cooling. Six neurones were observed in decerebrate rats with properties apparently identical to those in intact rats.

7. We conclude that the switching response of thalamic and hypothalamic scrotal temperature-responsive neurones is probably generated in the nucleus raphe magnus and passed in parallel to the thalamus and hypothalamus. In addition, thalamic neurones depend on an intact link with the cerebral cortex for the generation of their switching responses.

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