The role of endogenous corticoids in fever responses caused by recombinant murine interleukin (IL)-1 beta and IL-6 was studied in adult male Wistar rats. Adrenalectomy diminished the development of fever after intracerebroventricular (icv) injection of these ILs and lowered body temperature. Intraperitoneal administration of the same doses of ILs did not produce fever in intact animals or hypothermia in adrenalectomized rats, thus suggesting a central site of action of IL-1 beta and IL-6 in these experiments. Chronic replacement with moderate doses of corticosterone restored the fever response in adrenalectomized animals in response to icv administration of IL-1 beta but only partially reversed the fever caused by IL-6. Adrenalectomized animals acutely treated with corticosterone and thereafter with either IL-1 beta or IL-6 developed fever more rapidly than did chronically corticosterone-treated animals. In intact animals corticosterone blocked the fever response to icv injected IL-1 beta. We propose that in the rat corticosterone acts in a bimodal manner on body temperature; it exerts a permissive central effect on the fever response and limits the production of inflammatory mediators in the brain. Conversely, higher corticosterone doses probably reduce the magnitude of the fever response.