Expression of human inducible nitric oxide synthase (hiNOS) is under cytokine control and is transcriptionally regulated. The hiNOS and mouse iNOS (miNOS) genes are regulated differently by cytokines. To understand better the transcriptional regulation of the hiNOS gene, the 8.3-kb hiNOS promoter was characterized. Promoter activity was evaluated by transient transfection of hiNOS luciferase constructs in A549 human alveolar type II epithelium-like cells in the presence and absence of cytokines (IFN-gamma, IL-1 beta, and TNF-alpha). Important cytokine-responsive elements are located at -3665 to -5574 bp (containing two perfectly matched AP-1 sites which are not present in miNOS promoter) and -8093 to -8296 bp (one perfectly matched NF-kappa B site) of the hiNOS promoter region. Likely, these two AP-1 sites and the upstream NF-kappa B site are important in the transcriptional induction of hiNOS by cytokines. Our data demonstrate the molecular basis for the different cytokine-stimulated characteristics of hiNOS and miNOS genes.