Fever developing after intracerebral injections of lipopolysaccharide (LPS) to guinea-pigs were monophasic, with only one peak of inner body temperature,slowly developing and longlasting in a dose range 20 to 200 ng of LPS. Latency time was inversely related to the dose of LPS. Indomethacin injected to the third brain ventricle did not abolish fever response.