Acute ethanol administration temporarily decreases the sensitivity to endotoxin (lipopolysaccharide, LPS) in the liver. The purpose of this study was to investigate the changes of toll-like receptor (TLR)-4, a newly identified LPS receptor in macrophages, in the liver following acute ethanol administration. Male C57BL/6N mice were given a bolus intragastric administration of ethanol (5 mg/g BW) through a gastric canula, and liver samples were obtained 2-48 h later. RAW264.7 macrophages were cultured in the presence of ethanol (100 mM) or LPS (10 ng/ml) for up to 4 h. TLR-4 mRNA in the liver and RAW264.7 cells was detected by RNase protection assay. As expected, TLR-4 mRNA was clearly detected in the control liver; however, it was barely detectable in the liver 2-6 h after ethanol administration, followed by the gradual increase to the basal levels 48 h later. Interestingly, LPS (10 ng/ml), but not ethanol (100 mM), decreased TLR-4 mRNA in RAW264.7 macrophages in 4 h. Indeed, gut-sterilization by oral antibiotics pretreatment prevented the decrease in TLR-4 mRNA caused by acute ethanol administration, supporting the hypothesis that gut-derived endotoxin is involved in the mechanism. These findings clearly indicated that acute ethanol administration in vivo down-regulates TLR-4 expression in the liver. This phenomenon most likely explains the mechanism by which acute ethanol blunts the response of Kupffer cells to LPS transiently.