Chronic liver disorders represent a serious health problem, considering that 300 million people worldwide are hepatitis B virus carriers, and 8,000–10,000 patients per year, in the U.S. alone, die as a result of liver failure caused by hepatitis C infection. Nitric oxide synthase (NOS) regulates hepatic vasculature; however, the patterns of expression and activity of NOS proteins in healthy and diseased human livers are unknown. Sections of diseased (n = 42) and control livers (n = 14) were collected during orthotopic liver transplants and partial hepatectomy. The diseased sections included alcoholic cirrhosis, viral hepatitis, cholestasis, acute necrosis, and uncommon pathologies including α₁-anti-trypsin disorder. The endothelial NOS (eNOS), inducible NOS (iNOS), and neuronal NOS (nNOS) were studied by using the citrulline assay, Western immunoblot, immunohistochemistry, and in situ hybridization. The systemic generation of plasma NO metabolites was measured by HPLC. In control livers, Ca²⁺-dependent and –independent NOS activities were identified by Western analysis as eNOS and iNOS, respectively. The eNOS was uniformly distributed in the hepatocytes and also detected in the endothelium of hepatic arteries, terminal hepatic venules, sinusoids, and in biliary epithelium. The iNOS was detected in hepatocytes and localized mainly in the periportal zone of the liver acinus. This pattern of distribution of eNOS and iNOS in normal liver was confirmed by in situ hybridization. In diseased livers, there was a significant increase in Ca²⁺-independent NOS with the corresponding strong appearance of iNOS in the cirrhotic areas. The eNOS was translocated to hepatocyte nuclei. Thus, eNOS and iNOS proteins are differentially expressed in healthy human liver, and this expression is significantly altered in cirrhotic liver disorders.