Although urinary trypsin inhibitor (UTI) has been shown to inhibit tumor necrosis factor- (TNF-) production, the detailed mechanism(s) remains unclear. This study was undertaken to elucidate the molecular mechanism(s) underlying this inhibitory effect in monocytes in vitro and in rats given lipopolysaccharide (LPS). TNF- production by monocytes stimulated with LPS (100 ng/mL) was inhibited by UTI at concentrations higher than 100 U/ml. Expression of early growth response factor-1 (Egr-1) and phosphorylation of extracellular signal-regulated protein kinases (ERK)1/2 in monocytes stimulated with LPS were inhibited by UTI. UTI (50,000 U/kg, iv) inhibited LPS (5 mg/kg, iv)-induced increases in lung tissue levels of Egr-1, TNF- mRNA, and TNF- in rats. UTI inhibited LPS-induced hypotension by inhibiting pulmonary induction of inducible nitric oxide synthase (iNOS). We previously demonstrated that anti-TNF- antibody and aminoguanidine, a selective inhibitor of iNOS, reduced LPS-induced hypotension in this animal model. Furthermore, we also reported that reduction of LPS-induced coagulation abnormalities in rats did not affect inflammatory responses and hypotension in this animal model. Taken together, these observations strongly suggested that UTI inhibited LPS-induced production of TNF- by inhibiting activation of the ERK1/2-Egr-1 pathway in monocytes, which might at least partly contribute to reduction of hypotension through inhibition of iNOS induction in rats given LPS.