In unanesthetized rats, the effects of intracerebroventricular injections of prostaglandin E1 (PGE1) and cholecystokinin octapeptide (CCK-8) on body temperature (Tb) regulation were studied. Both PGE1 and CCK-8 evoke hyperthermic responses of short latency and duration. Each substance raises the temperature, irrespective of initial temperature (Tbi). However, the maximal change in Tb and the rate of Tb rise depends on Tbi. A similarity of PGE1 and CCK-8 central thermoregulatory effects and putative roles of these substances in the mechanisms of fever, are discussed.