The thermosensitivity of anterior hypothalamic-preoptic neurons was studied in cats during the waking-sleeping cycle. Direct cooling and warming of the anterior hypothalamic-preoptic region was accomplished with water-perfused thermodes. Neuronal thermosensitivity was determined by means of the linear regression analysis of firing rate changes vs anterior hypothalamic-preoptic temperature changes. A total of 117 neurons were classified as thermosensitive during wakefulness and synchronized sleep (20.1% of the studied neurons). Cold-sensitive neurons outnumbered warm-sensitive neurons by 3.7:1. The homeothermic states, wakefulness and synchronized sleep, are characterized by similar frequency distributions of neuronal thermosensitivity, although variable changes in single neuron thermosensitivity are state-dependent. Such changes underlie the quantitative differences in homeothermic regulation between these states. The impairment of thermoregulation during desynchronized sleep is characterized by a different frequency distribution of neuronal thermosensitivity resulting from both a drop in the responsiveness to thermal stimulation of a majority of neurons and a reversal in the sensitivity to cooling and warming of a minority of neurons. In conclusion, only the frequency distribution of thermosensitivity in the neuronal population is indicative of changes in the thermoregulation paradigm across behavioral states.