Thermosensitive anterior hypothalamic neurons (pre-optic region) were studied in urethane and chloralose anesthetized cats in an attempt to characterize the hypothermic action of delta 9-THC at the neuronal level. One hundred and seventy-eight single neurons were isolated and subjected to thermal challenge, 66 were found to reproducibly alter firing frequency at a significant level (thermosensitivity (T.S.) greater than 0.75). Twenty-one of these units met the criteria for primary thermodetectors, 34 were heat-sensitive interneurons, and 11 were cold-sensitive interneurons. Administration of delta 9-THC (1.0-2.0 mg/kg i.v.) decreased the spontaneous firing and increased the T.S. of the primary thermodetector units. delta 9-THC also increased the spontaneous firing frequency as well as the T.S. of heat-sensitive interneurons, while decreasing both the T.S. and spontaneous firing of cold-sensitive interneurons. The decreased spontaneous firing of primary thermodetectors could result from altered facilitory or inhibitory influences converging on these cells. The increased thermosensitivity is consistent with the hypothesis that the pre-optic region modulates cannabinoid-induced hypothermia.