Interferons possess important antifibrotic properties. In addition, there is evidence that they induce the production of nitric oxide (NO) and that it downregulates the synthesis of extracellular matrix by certain cells. The aim of the present work was to evaluate if L-arginine, the NO synthase substrate, is able to increase the antifibrotic properties of interferon-α 2b and if L-NAME, an NO synthesis inhibitor, can prevent them. Fibrosis was induced by bile duct ligation (BDL) for 5 weeks in rats and interferon-α 2b (IFN; 100,000 IU rat, s.c., daily) and/or L-arginine (500 mg/kg, p.o., twice daily) or L-NAME (100 mg/kg, p.o., twice daily) were administered. Collagen content was determined by measuring hydroxyproline in liver samples. Malondialdehyde (MDA) was used to estimate lipid peroxidation levels. Glycogen was measured colorimetrically. Serum enzyme activities and bilirubins were determined by standard procedures. Fibrosis was increased 6-fold by BDL. L-arginine or IFN partially prevented the increment in collagen. Furthermore, administration of both drugs simultaneously showed an additive effect (P < 0.05), while L-NAME abolished the protective effect of IFN. The same effect was observed on the other markers of liver function or damage studied herein. The additive effects of L-arginine and IFN could be due to a synergism of both compounds by increasing NO concentration, which can act as an antifibrotic agent but also as a cytoprotective compound. These results also suggest that the protective effects of IFN are mediated by NO, since L-NAME prevented them.