Shock states induce the expression of inducible nitric oxide synthase (iNOS) in both Kupffer cells and hepatocytes in the liver, but little is known about its subcellular localization in these cells. Studies were undertaken to characterize the subcellular location of iNOS in hepatocytes in response to sepsis. By immunofluorescence analysis, intraperitoneal challenge with bacterial lipopolysaccharide induced cytosolic iNOS in Kupffer cells but punctate labeling in hepatocytes. Cultured rat hepatocytes exposed to interferon gamma, interleukin 1, and tumor necrosis factor ␣ showed iNOS protein expression within peroxisomes as early as 4 hours after stimulation, as determined by colabeling for catalase or PMP70. To a lesser extent, iNOS was also observed associated with the plasma membrane and in undefined intracellular aggregates. S tudies in the past decade have shown that all nitric oxide synthases (NOS), whether constitutive or inducible, are regulated at transcriptional, posttranscriptional, translational, and posttranslational levels. 1 One important aspect of NOS regulation is its subcellular localization. For example, endothelial NOS is localized to the caveolae of endothelial cells, where direct association with caveolin I regulates enzyme activity. 2 Such localization may enhance NO function by concentrating NO release close to the cell surface. The inducible NOS (iNOS; NOS2) isoform, which contributes to inflammatory processes through high NO output, is regulated positively and negatively by cytokines but can also be controlled posttranslationally by NO and/or its reaction products. 1,3,4 Furthermore, this enzyme is found in both particulate and cytosolic cellular pools. 5 Our interest has focused on the regulation and function of iNOS in the liver. This enzyme is readily upregulated in hepatocytes, 6 and human iNOS expression was first detected in hepatocytes. 7 In vivo studies have shown that the consequences of iNOS up-regulation in liver are dependent on the physiologic or pathologic circumstances. Following conditions associated with severe redox stress, iNOS contributes to hepatocellular damage during hemorrhagic shock or hepatic ischemia/reperfusion. 8,9 In contrast, iNOS-generated NO suppresses hepatocyte apoptosis in endotoxemia 10 and following cytokine exposure. 11 To better understand how iNOS functions within hepatocytes as well as factors that might determine the fate of NO in the liver, we undertook a study to characterize its subcellular localization. We show that iNOS localizes to the peroxisome, plasma membrane, and cytoplasm. Interestingly, increased expression of iNOS is associated with decreased catalase expression.