The effects of electrical stimulation of the ventral subiculum (VSB) of hippocampal formation on the thermosensitivity of neurons in the preoptic area and the septum (PO/SP) were studied in the urethane-anesthetized rat. The local thermosensitivity of sixteen of 28 PO/SP thermosensitive neurons (25 warm-units and 3 cold-units) was reduced or lost during low frequency (4.8 Hz) stimulation of VSB. The extent of reduction was positively correlated to the magnitude of inhibition by the VSB stimulation. Thermosensitivity did not change in 11 of the remaining 12 warm-units and increased slightly in one warm-unit during VSB stimulation. In the conscious rat, bilateral stimulation (5 Hz) of VSB for 10 min caused a rise in rectal temperature of 0.28-0.87 degrees C in a warm environment (30-33 degrees C) and a fall of 0.18-0.57 degrees C in a cold (9.4-9.8 degrees C) environment. The results suggest that the predominantly inhibitory input from VSB decreases the thermosensitivity of PO/SP warm- and cold-sensitive neurons and thereby reduces the ability of thermoregulation.