Local warming of the medial preoptic/anterior hypothalamus (POAH) promotes sleep, enhances EEG slow-wave activity during sleep, and suppresses arousal-related discharge in neurons of the midbrain reticular formation (MRF) and the posterior lateral hypothalamic area (PLHa). Another important site of sleep and arousal regulation, and a potential site of POAH thermal modulation, is the magnocellular basal forebrain (BF). We examined the ability of local POAH warming during wakefulness to influence the spontaneous and evoked discharge of neurons recorded in the BF of unanesthetized, unrestrained cats. Seventy of 174 BF neurons responded to 60-90 s periods of POAH warming with either increases or decreases in discharge rate. Forty-one of the 70 responsive cells displayed suppression of waking discharge during warming. Discharge rate in these cells declined by an average of 26.04 +/- 2.76%/degrees C of POAH temperature increase. The majority of warming-suppressed BF cells (73%) displayed higher rates of discharge during periods of wakefulness compared to periods of sleep. Twenty-nine of 70 responsive cells responded to POAH warming with an average increase in discharge rate of 43.81 +/- 6.26%/degrees C. A majority of these neurons (62%) exhibited higher spontaneous discharge rates during sleep compared to waking. Orthodromic excitatory responses were evoked in 29 BF cells by electrical stimulation of the MRF or PLHa. Thirteen of 29 cells displayed a waking-related discharge pattern, and responded to POAH warming with a significant suppression of evoked excitation. For a group of 15 behavioral state-indifferent cells (i.e., cells displaying no modulation of spontaneous discharge rate across the sleep-waking cycle), POAH warming had no effect on evoked excitatory responses. These results support the hypothesis that thermosensitive neurons of the POAH exert control of sleep-waking state, in part, via modulation of arousal- and sleep-regulating cell types within the magnocellular BF.