At there ambient air temperature range, the rectal temperature changes following infusion of histamine either into lateral ventricle (L.V.) or IVth ventricle (IVth V) were studied. At an ambient temperature range of 19-22 degrees C, hypothermia occurred following histamine infusion either into L.V. or IVth V. Hypothermia elicited from infusion of histamine into L.V. was prevented with pretreatment of H1-receptor blocker (mepyramine), but in case of IVth V, it was prevented with H2-receptor blocker(cimetidine). These H1 and H2-receptor antagonists were ineffective in preventing hypothermia following histamine infusion into either L.V. or IVth V, when the ambient air temperature was maintained low (11-13 degrees C).