In critically ill patients, the human inducible nitric oxide synthase (iNOS) gene is expressed in nearly every organ and has been shown to be associated with the refractory hypotension of septic and hemorrhagic shock. The molecular regulation of iNOS expression is complex and occurs at multiple sites in the gene expression pathway. Work in our laboratory has demonstrated that a combination of cytokines synergistically activate iNOS expression, which has resulted in the cloning of the first human iNOS gene from cytokine-stimulated hepatocytes. In addition, we have demonstrated that iNOS expression is transcriptionally regulated and that the functional promoter elements are located upstream of -4.7 kilobases (kb) within a unique enhancer region containing four functional nuclear factor kappa B elements. These results contrast markedly with the murine iNOS promoter, where only 1.0 kb of 5′-flanking sequence is required for lipopolysaccharide and cytokine responsiveness. Furthermore, numerous mechanisms have evolved to downregulate iNOS expression. By elucidating these mechanisms, therapeutic strategies to govern iNOS expression may be developed.