Treatment of mice with Corynebacterium parvum induces chronic inflammation. This treatment followed by an injection of lipopolysaccharide (LPS) produces hepatic necrosis and death. We examined liver tissue by using electron paramagnetic resonance (EPR) spectroscopy and found that, in addition to the previously reported nonheme nitrosyl complexes, heme nitrosyl complexes were also formed. Hemoglobin nitrosyl complexes measured in the whole blood of mice treated with C. parvum were not increased after additional LPS treatment. However, this treatment significantly increased the heme nitrosyl complexes in the liver, whereas the nonheme nitrosyl complex concentration was unaffected. EPR signals from whole blood and liver tissues from mice treated with C. parvum and C. parvum + LPS were inhibited by prolonged treatment with NG-monomethyl-L-arginine (L-NMA). Nitric oxide (.NO) is known to bind to cytochrome P450 heme, and we consistently found a suppression of EPR signals attributable to ferric low-spin cytochrome P450/P420 peaks in the livers of mice treated with C. parvum and C. parvum + LPS. By performing analyses of EPR spectra obtained from hepatocytes exposed to .NO, we were able to unambiguously identify EPR signals attributable to cytochrome P420 and nonheme nitrosyl complexes in the livers of both treatments. Deconvolution of the composite in vivo EPR spectra indicated that hemoglobin nitrosyl complexes contributed weakly in the C. parvum livers, but threefold more in the C. parvum + LPS livers, suggesting that hemorrhage may have occurred. Experiments with L-NMA treatment revealed that this additional .NO production did not correlate with hepatic necrosis and onset of death. Immunoprecipitation of liver cytosols from C. parvum- and (C. parvum + LPS)-treated mice using an antibody against mouse inducible nitric oxide synthase showed that this enzyme was indeed present in the cytosolic fractions and was absent in those from control livers. Our novel detection of cytochrome P420 nitrosyl complex in vivo may be linked to any role of hepatic P450's functions during liver inflammation.