BACKGROUND Nitric oxide has a wide variety of homeostatic and pathological effects. Control of the production of nitric oxide by the inducible form of the enzyme resides in the 5' promoter region of the gene. Although control of the murine isoform has been investigated, little is known about the functional aspects of the human analog. METHODS A 3.9-kb 5' nontranslated region of the human gene was cloned, sequenced, and several reporter constructs prepared. The promoter-reporter constructs were transfected into human or murine monocytoid cells and reporter expression quantified following cytokine activation of the cells. The production of nitric oxide was also monitored. RESULTS Although a murine promoter-reporter functioned efficiently in both human and mouse cells, the human constructs functioned only in human cells. The activity of the mouse construct increased progressively with the addition of activating cytokines, but the human promoter-reporter did not. Although interleukin 1 beta drove expression of the human inducible nitric oxide synthase reporter, actual expression of nitric oxide required both interleukin 1 beta and interferon-gamma. CONCLUSIONS The data indicate that despite the significant homology between the human and mouse inducible nitric oxide synthase promoter sequence, control of the two genes is quite different. In addition to being more efficient in promoter activity, the murine promoter responds increasingly to cytokines that are not effective for the human analog. It is also apparent that human inducible nitric oxide synthase is controlled at both the level of transcription and post-translationally.