Skin temperature was increased at slow and rapid rates while single unit activity was recorded in the medial preoptic/anterior hypothalamus (POAH) of the unanesthetized rabbit. Of 69 units tested with slow skin heating, activity increased in 14, decreased in 38, and was unchanged in 17. The responsive units exhibited either an immediate or delayed change in firing rate during skin heating. During rapid skin heating, 6 of 19 units which could be tested underwent a qualitative change from their thermal responsiveness during slow skin heating. Rapidly responding neurons could provide an instantaneous indication of changes in skin temperature, whereas the activation of slowly responding units requires a relatively long period of skin heating. Qualitative shifts in thermal responsiveness during rapid skin heating could provide an additional mechanism for detecting abrupt changes in ambient temperature. Thus, POAH thermal responsive neurons provide input regarding both steady state and rapid changes in skin temperature.