The stress of physical restraint has been shown to cause an elevation in the body temperature of rats. In this study, we compared the febrile responses of restrained and unrestrained animals to the injection of interleukin-1 beta (IL-1 beta) and lipopolysaccharide (LPS). We found that, prior to injection, the body temperature of the restrained animals was about 1 degree C higher than that of the unrestrained rats. The restrained rats showed significantly smaller increases in body temperature in response to the injection of either pyrogen than did the unrestrained animals.