Central anionic influences on the regulation of body temperature were studied in 42 conscious male rats. The animals were divided into seven equal groups and were given intraventricular infusions of either chloride or bicarbonate solution of sodium, calcium, or potassium. Infusions were made in the unanesthetized and unrestrained animals through stainless steel cannulae, chronically implanted into the anteroventral part of third ventricle. Control rats received intraventricular infusions of artificial cerebrospinal fluid. All of the chloride solutions, irrespective of the associated cations, elicited hyperthermia, whereas bicarbonates had hypothermic effect. Responses of chloride and bicarbonate solutions varied significantly (p < 0.001). There was, however, cationic modification of the anionic responses. Thus, sodium ions manifested hyperthermic modifications, accentuating hyperthermia of chloride and attenuating hypothermic effect of bicarbonate. Calcium and potassium ions exerted hypothermic modulation. The results suggest that anionic ioncentration of intraventricular CSF is crucial for central regulation of body temperature in unanesthetized conscious rats. The cations probably have only modulatory influences.