Bacterial lipopolypolysaccharide (LPS)-induced fever involves induction of the proinflammatory cytokines interleukin (IL)-1 alpha, IL-1 beta, tumor necrosis factor-alpha (TNF-alpha), and IL-6, both in the periphery and in the brain. These molecules can induce expression of each other and also regulate expression of their own receptors in a complex manner. The functional hierarchy of these highly inducible proteins is therefore difficult to determine. Using mice strains carrying the null mutations of IL-1 beta, IL-1RI, IL-1RAcP, or IL-6, respectively, we show that LPS-induced fever involves IL-1 beta, which acts at a complex consisting of the type I IL-1 receptor and the IL-1RAcP. This action occurs prior to central IL-6 release, which has been shown to be a necessary component of fever responses induced by LPS, IL-1 beta, and also TNF-alpha. In the absence of IL-1 beta, as in IL-1 beta-deficient mice, LPS, IL-1 alpha, and IL-1 beta cause hyperresponsive fevers when exogenously applied. Murine TNF-alpha is a poor pyrogen in mice even when mice are kept at thermoneutral temperature (30 degrees C). TNF-alpha-mediated fever depends on central IL-6 expression.