The role of nitric oxide (NO) in liver injury and fibrosis is unclear. The purpose of this study was to determine whether inducible NO synthase deficiency (iNOS−/−) affects liver injury and fibrosis produced in mice by chronic carbon tetrachloride (CCl4) administration. Wild-type (WT) or iNOS−/− mice were subjected to biweekly CCl4 injections over 8 weeks, whereas controls were given isovolumetric injections of olive oil. Serum aminotransferases were lower after CCl4 in the iNOS−/− than in the WT mice, which correlated with decreased necrosis on liver histology. There was increased apoptosis, a lower number of stellate cells, and a lesser degree of fibrosis after CCl4 in the iNOS−/− as compared with the WT mice. α1(I) collagen messenger RNA (mRNA) was markedly increased after CCl4 in the WT and to a significantly lesser extent in the iNOS−/− mice. Liver matrix metalloproteinase-9 (MMP-9) mRNA and MMP-2 mRNA were increased more in the WT than in the iNOS−/− mice after CCl4. Also tissue inhibitor metalloproteinase 1 (TIMP-1) mRNA was increased to a much greater extent in the WT than in the iNOS−/− mice after CCl4 (P< 0.05). However, MMP-9 and TIMP-1 protein, determined by western blot, were similarly increased after CCl4 in both groups of mice.Conclusion:NO protects against CCl4-induced apoptosis. In the absence of iNOS, there is decreased necrosis, increased apoptosis, and reduced liver fibrosis.