The endothelium of the lung is sensitive to the toxic effects of oxygen, and early evidence of toxicity is characterized by protein leak and extravasation of red blood cells. The overproduction of oxygen free radicals plays a critical role in the pathophysiology of a hyperoxic lung injury. Recently, heme oxygenase 1 (HO-1), the rate-limiting enzyme in the metabolism of heme, has been found to have a protective role in oxidant injury. Our laboratory and others have identified HO-1 as a hyperoxia-inducible protein. In this study, we characterized HO-1 expression and evaluated its regulation in human pulmonary endothelial cells. Hyperoxia results in a relatively small increase in HO-1 expression; however, this induction is potentiated by heme and dramatically potentiated in the presence of free iron. This is probably more reflective of the in vivo situation in which there is extravasation of heme and iron products. We also found that HO-1 expression depended on chelatable iron. The iron chelator desferrioxamine not only inhibited the iron-dependent potentiation of HO-1 in response to hyperoxia but also inhibited both hyperoxia and basal expression. On the basis of inhibitor studies and nuclear run-on assays, we demonstrated that this induction is transcriptionally dependent. We also evaluated 4.5 kb of the human HO-1 promoter region and demonstrated that this region has promoter activity to the stimulus heme; however, there was no evidence of promoter activity to either iron or hyperoxia. This diversity of promoter activity to heme, heavy metals, and hyperoxia is unique to the human HO-1 gene.