Aging is associated with a blunted or absent fever response to naturally occurring infections or to the peripheral administration of bacterial products and proinflammatory cytokines, including interleukin-1beta (IL-1beta). Whether old rats also exhibit an attenuated fever response when challenged with direct brain administration of IL-1beta is unknown. Here we investigated the fever response of young (3-5 mo) and old (24-26 mo) Long-Evans rats to the intracerebroventricular microinfusion of IL-1beta. Core body temperature was monitored by telemetry in freely moving rats. Intracerebroventricularly administered IL-1beta induced comparable increases in body temperature in young and old Long-Evans rats. In the two groups, IL-1beta-induced fever was similar both in latency to peak fever and maximal fever response, whether the cytokine was administered 2 h after lights on or just before lights off. These data show that old Long-Evans rats are not defective in their capacity to develop a fever in response to brain administration of IL-1beta.