The relationship among gender, lipopolysaccharide (LPS), and liver disease is complex. Accordingly, the effect of estrogen on activation of Kupffer cells by endotoxin was studied. All rats given estrogen intraperitoneally 24 h before an injection of a sublethal dose of LPS died. Mortality was prevented totally by pretreatment with gadolinium chloride, a Kupffer cell toxicant. Peak serum tumor necrosis factor-โฃ (TNF-โฃ) values as well as TNF-โฃ mRNA in the liver after LPS were twice as high in the estrogen treated group as in the untreated controls. Plasma nitrite levels and inducible nitric oxide synthase in the liver were also elevated significantly in estrogen-treated rats 6 h after LPS. Furthermore, Kupffer cells isolated from estrogen-treated rats produced about twice as much TNF-โฃ and nitrite as controls did in response to LPS. In addition, Kupffer cells from estrogen-treated rats required 15-fold lower amounts of LPS to increase intracellular Ca 2ุ than controls did, and Kupffer cells from estrogen-treated animals expressed more CD14, the receptor for LPS/LPS binding protein, than controls. Moreover, estrogen treatment increased LPS binding protein mRNA dramatically in liver in 6-24 h. It is concluded that estrogen treatment in vivo sensitizes Kupffer cells to LPS, leading to increased toxic mediator production by the liver.