In the unanesthetized rat, Ca++ ions in solutions ranging from 2.6 to 112.0 mM in excess of the normal level in CSF were applied at different sites in the brain and by three separate procedures, Colonic temperature was monitored and in certain experiments, the amount of food pellets and water consumed was measured simultaneously following the administration of excess Ca++ ions. An infusion into the lateral cerebral ventricle of excess calcium in a volume of 5.0 mul produced a concentration-dependent hypothermia, This fall in temperature was not attenuated by a prior intraventricular infusion of mecamylamine and often enhanced by atropine. Depending on the site, a microinjection of excess Ca++ ions in a volume of 0.5 to 1.0 mul directly into the hypothalamus produced hypothermia or feeding. The sites of maximum sensitivity at which excess calcium caused a decline in temperature were clustered in the caudal hypothalamus, whereas those at which calcium elicited feeding were distributed widely in caudo-lateral, medial and rostral hypothalamic areas. Push-pull perfusions at a rate of 20 to 25 mul per min for 10 to 20 min at homologous sites caused similar responses but the cation concentration required to evoke feeding or hypothermia was significantly less than that of either microinjection or intraventricular infusion. These findings demonstrate species continuity in the rat concerning anatomical localization of the postulated set-point mechanism for body temperature. Several different pathways in the feeding system are affected by an alteration in the hypothalamic level of calcium.