1. Responses of single lumbar dorsal horn units to controlled noxious radiant heating of glabrous hindfoot skin were recorded in cats anesthetized with sodium pentobarbital and 70% N2O. The heat-evoked responses of all units studied were markedly suppressed during concomitant electrical stimulation (mean, 30 Hz; 25-300 microA) of medial preoptic and ventromedial septal areas. 2. Brain sites at which stimulation inhibited spinal neuronal heat-evoked responses were mapped by systematically varying the depth of the stimulating electrode in tracks at anteroposterior levels +14 through +18. At each stimulation site, the magnitude of the spinal neuronal response to heat (50 degrees C, 10 s, 1 per 3 min) during brain stimulation was expressed as a percentage of the control response (no brain stimulation), which was stable in size over repeated trials. Sites at which stimulation markedly reduced the heat-evoked response were located in the medial preoptic area and in the ventromedial septum (diagonal band of Broca) up to anterior level +17. 3. The magnitude of inhibition increased with graded increases in brain-stimulation intensity. For 15 units, the mean current threshold to generate inhibition was 25 microA. 4. Responses of dorsal horn neurons to a series of graded noxious heat stimuli increased linearly from threshold (40-45 degrees C) to 52 degrees C. The slopes of such linear temperature-response curves were significantly reduced, without a change in the response threshold, when the temperature series was repeated during concomitant preoptic or septal stimulation. 5. The possible relationship of the medial preoptic and septal areas to inhibitory systems in the brain stem, and their possible role in analgesic mechanisms, are discussed.