Polymyxin B, a cyclic cationic polypeptide antibiotic, binds to the lipid A of bacterial endotoxin (lipopolysaccharide; LPS) to inhibit LPS-induced fever. On the basis of a casual observation, we hypothesised that in rats (unlike in rabbits and goats), intravenous (i.v.) polymyxin B would decrease resting body temperature. A single i.v. injection of polymyxin B (10, 100 or 1000 microg/kg) induced a rapid, marked drop in body temperature in a dose-related manner, with no change in physical activity. However, the highest dose (1000 microg/kg) seemed to impair heat-loss mechanisms and/or functions controlling the animal's day-night cycle [because the day-time body temperature remained elevated for two days after the injection (versus the pre-injection level)]. By contrast, rats given 100 or 10 microg/kg of the drug showed a normal day-night cycle after recovery from the initial hypothermic effect of the drug. Therefore, we used the middle dose of polymyxin B (100 microg/kg) in the subsequent experiments. In these experiments, significant decreases in metabolic rate and heat-loss responses were observed immediately after an i.v. injection of polymyxin B (100 microg/kg). By contrast, intracerebroventricular injection of polymyxin B (3 microg) had no effect on resting body temperature. These results suggest that the observed decrease in metabolic rate is responsible for the polymyxin-B-induced hypothermia. Further, rats may react with a reduction in heat-loss responses so as to prevent the body temperature decreasing too far in response to polymyxin B. Thus, polymyxin B modulates or interferes with the peripheral mechanisms underlying body temperature regulation in rats.