1. The firing frequency of polymodal nociceptor units with C-fibre axons was found to increase exponentially with skin temperature as the latter was raised linearly at 1 degree C/sec. Q10 values ranged from 3.6 to 120. 2. Following skin heating to 54--64 degrees C, average firing thresholds fell substantially. This sensitization was accompanied by a 46% increase in interval by interval variability of firing during heat stimulation. 3. Stopping the blood flow to the saphenous area by ligaturing the femoral or saphenous arteries had no significant effect on heat thresholds, on firing patterns during heat stimulation or on variability of firing in sensitized units. There were no changes in continuous, background firing during periods of arterial ligation. 4. It is concluded that if the maintenance of nociceptor heat sensitization depends on the continuous local production of chemical substances, these substances must either be rapidly broken down locally or able to move only slowly into blood vessels.