Nitric oxide (NO) synthesis is upregulated during chronic hepatic inflammation. The present study characterized the mechanisms involved in the induction of NO production and inducible NO synthase (iNOS) messenger RNA (mRNA) expression in murine embryonic liver cell line, BNL CL.2 cells. No production by BNL CL.2 cells was induced by interferon-r (IFN-r) plus lipopolysaccharide (LPS). However, other inflammatory cytokines such as interleukin (IL)-β, tumor necrosis factor α (TNF-α), and IL-6 had no additional effects on it. The stimulatory effects of IFN-r and LPS were time- and dose-dependent. NO secretion was inhibited by treatment with inducible NOS inhibitors such as NG-monomethyl L-arginine, NG-amino-L-arginine, and diphenylene iodonium. iNOS mRNA was induced 3 hours after IFN-r plus LPS treatment, and iNOS expression was maximal in the presence of IFN-r and LPS. The protein tyrosine kinase inhibitors such as genistein and tyrphostin reduced IFN-r plus LPS-induced iNOS mRNA expression and NO production. In contrast, the inhibitors of protein kinase C, protein kinase A, and protein phosphatases did not affect iNOS expression induced by IFN-r plus LPS. In addition, iNOS mRNA expression was completely blocked by treatment with tyrphostin. However, mRNA expression of an early response gene, JunB, and constitutively expressed genes β-actin and GAPDH were not inhibited by tyrphostin. Furthermore, tyrphostin inhibited the promoter activation of iNOS gene induced by IFN-γ plus LPS, and it also suppressed IFN-γ plus LPS-induced nuclear factor-κ B-binding activity but not AP-1-binding activity. These results suggest that NO production and iNOS mRNA expression in this cell line is dependent on protein tyrosine kinases but does not require protein kinase C, protein kinase A, or protein phosphatases.