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1. In rats the scrotal temperature was raised or lowered with a water-perfused thermode while micro-electrode recordings were made of unit activity in the ventrobasal complex of the thalamus. The electrodes were aimed at the region where evoked responses had been found by electrical stimulation of the scrotum. Recording sites were marked by iontophoresis of dye from the micro-electrode.

2. Changes in firing rate of thalamic neurones were only found in the scrotal temperature range of 31-40° C. Within this range, 72% of the 123 cells tested were excited or suppressed by skin warming. At temperatures above or below this range, activity was not affected. Most of the cells responded just to temperature and only 7% were also excited by touch.

3. Raising temperature in the range 31-40° C caused 82% of the thermally responding cells to increase their firing rate and 18% to decrease their rate. Individual neurones showed a sudden and maintained change in their activity for scrotal temperature increases of only 2, 1 or even 0·5° C. Mean firing rates changed by factors of about 8 or more with these temperature increases and further warming did not change the rate. These step-like changes in firing rate were found at different points over the whole skin temperature range of 31-40° C, but most were between 33 and 38° C.

4. For a given neurone the step-like change in activity occurred once its critical temperature was reached, irrespective of whether this was achieved by a step increase of skin temperature over 1-2 sec or by a slow ramp increase lasting several minutes.

5. It is not possible to say whether the skin warm receptors, cold receptors or both were responsible for these thalamic responses, but the results do show that incoming thermal information is considerably processed when it reaches the thalamic level.