The skin temperature of the pinna was recorded in unrestrained cats sleeping at different ambient temperatures. The results show that during fast wave sleep vasomotion is inconsistent with homeothermic regulation as ear skin temperature increases and decreases at low and high ambient temperatures, respectively. The effects of preoptic heating during slow wave and fast wave sleep reveal that a depression of the responsiveness of hypothalamic thermoceptive structures underlies the absence of thermoregulatory vasomotor responses during fast wave sleep.