The purpose of this study was to test the hypothesis that attenuation of the fever response to lipopolysaccharide (LPS) following hemorrhage is accompanied by changes in serum glucocorticoid levels and a decreased bioactivity of TNF-αand IL-6 in plasma. Hemorrhage was induced in rats by the withdrawal of 20% of estimated total blood volume. LPS (50µg/kg) or saline were injected intraperitoneally immediately after the hemorrhage. Blood samples were taken 1.5 h for TNF-αbioactivity and corticosterone measurements and 5 h after treatment for IL-6 bioactivity. Body temperature (T<sub>b</sub>) was measured by biotelemetry. The 20% hemorrhage led to a significant reduction in hematocrit measured at 1.5 and 5 h after treatment. Furthermore, 20% hemorrhage caused a substantial elevation in serum corticosterone measured by radioimmunoassay at 1.5 h after treatment. This high concentration of corticosterone was not further potentiated by injection of LPS. Hemorrhaged rats treated with LPS responded with a markedly attenuated fever. Both TNF-αand IL-6 rises in the circulation due to LPS injection were significantly smaller in hemorrhaged rats compared to nonhemorrhaged LPS-injected rats. However, this degree of hemorrhage did not alter the T<sub>b</sub>or plasma TNF-αand lL-6 activity in hemorrhaged rats injected with saline. These results show that the inhibitory effect of hemorrhage on LPS-induced fever may be related to the decreased TNF-αand IL-6 activity in plasma. Hemorrhage-induced high level of corticosterone might contribute to the attenuation of fever, perhaps via the suppression of pyrogenic cytokines.