To examine the transcriptional regulation of the human cytochrome P450IA1 gene, a 3574 bp fragment containing 1140 bp of 5' flanking sequences, exon 1 (leader information only), intron 1, and the leader sequences from exon 2, was cloned upstream of the reporter gene, chloramphenicol acetyltransferase, and used to transfect the human hepatoma cell line, HepG2. In transient expression assays, treatment of the transfected cells with 3-methylcholanthrene, benzo-(a)pyrene or 2,3,7,8-tetrachlorodibenzofuran was shown to induce the expression of chloramphenicol acetyltransferase 10-fold. Previous studies by other investigators have identified a xenobiotic responsive element at >800 bp 5' to the cap site in the mouse and rat cytochrome P450IA1 gene. In the current report, deletion of sequences from the 5' side of the P450IA1 fragment, as well as internal deletions, were used to identify at least three additional regulatory elements. A second positive, 3-methylcholanthrene responsive element was localized to sequences between -49 and -560 in addition to confirming the location of a similar element between -831 and -1140. These elements flank a potent negative regulatory element that has been conserved between the rat, mouse and human P450IA1 genes and also exhibits significant sequence identity with one of the negative control elements of the human c-Ha-ras1 proto-oncogene. Deletion of the negative control element clearly demonstrated that the fragments containing xenobiotic responsive elements also possess positive, constitutive control activity. A fourth element located within intron 1 was shown to potentiate the activity of 3-methylcholanthrene when the cells were treated simultaneously with the glucocorticoid agonist, dexamethasone.