We have demonstrated that after intraperitoneal (i.p.) lipopolysaccharide (LPS) injection, old rats mount fevers similar to those of young rats at an ambient temperature (Ta) of 31°C, but not at 21°C. The same is true for i.p. or intravenous (i.v.) interleukin-1 administration. The underlying mechanism responsible for blunted fever in old rats may be a deficiency in communication between periphery and brain. Possibly, peripheral cytokine actions are altered in old rats such that the signal that reaches the brain is diminished. Here, we hypothesized that at standard laboratory temperatures not enough IL-1 is reaching the brain for fever to occur and that a warmer Ta would increase the influx of IL-1 into the brain, enabling old rats to generate fever. Young (3-5 mo) and old (23-29 mo) Long-Evans rats were maintained for 3 days at either Ta 21 or 31°C prior to i.v. injection with radiolabeled IL-1 to measure passage across the blood-brain barrier. Young rats showed similar influx of IL-1 into the brain at the two Tas but old rats showed significant influx only at the warmer Ta. These data suggest that the lack of fever at a cool Ta may be due to a reduced influx of IL-1 into the brain.