Medial hypothalamic stimulation produces analgesia to facial heating in unrestrained rats

Gradual heating of the face in unrestrained conscious rats produced a behavioural response at a mean threshold temperature of 41.9 degrees C (S.E.M. +/- 0.02, 174 tests). This temperature did not change with tests repeated at 5-min intervals over one hour. During stimulation of the preoptic area of the hypothalamus the response temperature was consistently raised by about 3 degrees C but returned to the control level within 5 min.