Macrophage inflammatory protein-1 (MIP-1) evokes an intense fever, independent of a prostaglandin mechanism, and is now thought to play an important role in the defence response to bacterial pyrogens. The purpose of this study was 2-fold: (i) to determine whether the potent doublet of this cytokine, MIP-1β, is actually produced in the brain in response to a pyrogenic dose of a lipopolysaccharide of Escherichia coli and (ii) to determine the anatomical site of synthesis of this cytokine in the brain. Following the intense fever produced by intraperitoneal administration of lipopolysaccharide in the unrestrained rat, MIP-1β immunoreactivity was identified post mortem in two regions of the brain implicated in fever: the organum vasculosum laminae terminalis (OVLT) and the anterior hypothalamic, preoptic area (AH/POA). Microinjection of goat anti-mouse MIP-1β antibody (anti-MIP-1β) directly into the AH/POA markedly suppressed fever in rats in response to lipopolysaccharide. Further, anti-MIP-1β administered 180 min after the injection of lipopolysaccharide acted as an antipyretic and reversed the fever induced by the endotoxin. Anti-MIP-1β or control immunoglobulin G antibody microinjected into the hypothalamus immediately before the intraperitoneal injection of the control saline did not alter the temperature of the rats. Taken together, the present results demonstrate that MIP-1β is produced in the brain in response to a bacterial endotoxin. These observations, in the light of earlier data on fever induced by MIP-1β, further support the hypothesis that endogenously synthesized MIP-1β acts as an intermediary factor in the evocation of fever by acting on the thermosensitive cells of the brain.