Lines of evidence suggested a possible link between leptin and hepatic fibrosis; however, whether leptin modulates the fibrogenesis in the liver remains unclear. The purpose of this study, therefore, was to evaluate the effect of leptin on inflammatory and profibrogenic responses in the liver caused by hepatotoxic chemicals. Male C57Bl/6 mice were given carbon tetrachloride (CCl(4)) (0.1 microL/g body weight [BW], intraperitoneally [IP]) and/or recombinant murine leptin (1 microg/g BW, IP) simultaneously, and sacrificed up to 72 hours later. Further, some mice were given thioacetamide (TAA; 200 microg/g BW, IP) and leptin 3 times per week for 4 weeks to evaluate the effect of leptin on chronic fibrogenic responses. A simultaneous injection of leptin enhanced acute CCl(4)-induced necroinflammatory and subsequent fibrotic changes in the hepatic lobules. The steady-state messenger RNA (mRNA) levels of alpha1(I) procollagen and heat shock protein 47 (HSP47) in the liver were potentiated when leptin was injected together with CCl(4). Expression of alpha smooth muscle actin (alpha-SMA) in the liver after CCl(4) treatment was also augmented markedly in combination with leptin. Further, leptin increased transforming growth factor beta1 (TGF-beta1) mRNA in the liver 24 hours after acute CCl(4) about 4-fold higher than CCl(4) alone. Moreover, leptin enhanced hepatic fibrosis and induction of alpha1(I) procollagen mRNA caused by chronic TAA administration. Collectively, these findings indicated that leptin augments both inflammatory and profibrogenic responses in the liver caused by hepatotoxic chemicals. It is postulated that the increase in systemic leptin levels enhances up-regulation of TGF-beta1, leading to activation of stellate cells, thereby augmenting the fibrogenic response in the liver.