Although thromboxanes (TXs), whose synthesis is regulated by cyclooxygenase (COX), have been suggested to promote inflammation in the liver, little is known about the role of TXA2 in leukocyte endothelial interaction during endotoxemia. The present study was conducted to investigate the role of TXA2 as well as that of COX in lipopolysaccharide (LPS)-induced hepatic microcirculatory dysfunction in male C57Bl/6 mice. We observed during in vivo fluorescence microscopic study that LPS caused significant accumulation of leukocytes adhering to the hepatic microvessels and non-perfused sinusoids. Levels of serum alanine transaminase (ALT) and tumor necrosis factor alpha (TNFα) also increased. LPS raised the TXB2 level in the perfusate from isolated perfused liver. A TXA2 synthase inhibitor, OKY-046, and a TXA2 receptor antagonist, S-1452, reduced LPS-induced hepatic microcirculatory dysfunction by inhibiting TNFα production. OKY-046 suppressed the expression of an intercellular adhesion molecule (ICAM)-1 in an LPS-treated liver. In thromboxane prostanoid receptor-knockout mice, hepatic responses to LPS were minimized in comparison with those in their wild-type counterparts. In addition, a selective COX-1 inhibitor, SC-560, a selective COX-2 inhibitor, NS-398, and indomethacin significantly attenuated hepatic responses to LPS including microcirculatory dysfunction and release of ALT and TNFα. The effects of the COX inhibitors on hepatic responses to LPS exhibited results similar to those obtained with TXA2 synthase inhibitor, and TXA2 receptor antagonist. In conclusion, these results suggest that TXA2 is involved in LPS-induced hepatic microcirculatory dysfunction partly through the release of TNFα, and that TXA2 derived from COX-1 and COX-2 could be responsible for the microcirculatory dysfunction during endotoxemia. (HEPATOLOGY 2004;39:139–150.)