The rate of transcription of the heme oxygenase gene is enhanced by a variety of agents including oxidants such as hydrogen peroxide and UVA (320-380 nm) radiation and the sulfhydryl reagent, sodium arsenite. To further analyze the inducible response, we have isolated genomic clones of the human heme oxygenase gene. A 1.44 kb fragment corresponding to a region extending from 1416 bp upstream of the mRNA cap site to 24 bp into the 5' untranslated region of the mRNA has been further subcloned and sequenced and used as the basis for the construction of recombinant CAT transient expression vectors. By deleting large portions of this fragment, we have established that elements within 121 bp of sequence immediately upstream of the mRNA cap site respond to various agents (sodium arsenite, hydrogen peroxide, hemin, cadmium chloride and 12-O-tetradecanoyl-phorbol-13-acetate) to give a 3- to 5-fold enhancement in transient expression of the reporter gene. Under the assay conditions employed, induction can only be detected when a SV40 enhancer element is present upstream of the promoter sequence. However, control experiments show that the SV40 sequences serve to amplify the response and are not directly involved in the induction itself. Only a small induction occurs when the entire 1.44 kb fragment is present. The results are consistent with the possibility that additional inducible enhancer elements lie outside of the sequence under study and that a silencer or negative regulatory element occurs upstream of the mRNA cap site within the 1.44 kb fragment