P>Pirfenidone (PFD; 5-methyl-1-phenyl-2(1H)-pyridone) is an effective and novel agent with antifibrotic and anti-inflammatory properties. In the present study, we investigated the antifibrotic effects of PFD on experimental liver fibrosis models in rodents and the possible underlying molecular mechanisms. Liver fibrosis was induced by carbon tetrachloride (CCl4) in BALB/c mice. Pirfenidone (250 mg/kg) and silymarin (50 mg/kg) were given to different groups of rats by gastric gavage for 4 weeks. Pirfenidone significantly attenuated fibrosis severity, as determined by histopathological scores and hydroxyproline levels in liver tissue, by 49.8 and 44.9%, respectively, compared with the CCl4-treated group. The antifibrotic effects of PFD were significantly greater than those of silymarin, as indicated by a decrease of 23.5 and 24.8% in histopathological scores and hydroxyproline levels, respectively. Liver fibrosis was also induced by albumin antigen-antibody complex in Wistar rats, which were then treated with the same doses of PFD and silymarin for 8 weeks. Pirfenidone significantly reduced the degree of fibrosis compared with CCl4-treated rats (by 45.0 and 51.0% as determined by histopathological scores and hydroxyproline levels in liver tissue, respectively). The antifibrotic effects of PFD were comparable to those of silymarin. The effects of PFD on the expression of extracellular matrix-associated genes in human hepatic stellate cells (the LX-2 cell line) were measured by real-time quantitative polymerase chain reaction. LX-2 cells were treated with or without 100 mu mol/L or 1 mmol/L PFD for 24 h. Pirfenidone significantly inhibited the expression of a-smooth muscle actin and Type I collagen in 8 ng/mL transforming growth factor-beta 1- or 5% fetal bovine serum-activated LX-2 cells in a dose-dependent manner. In conclusion, the results of the present study demonstrate that PFD is effective in ameliorating fibrogenesis induced by CCl4 in mice and by the albumin complex in rats. These effects were mediated mainly via inhibition of the activation of hepatic stellate cells, as well as antifibrotic actions (i.e. inhibition of collagen synthesis) of PFD.