Most if not all of the toxic responses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are mediated through the AhR, which requires ARNT to regulate gene expression. ARNT is also required by HIF-1alpha to enhance the expression of various genes in response to hypoxia. Since both the AhR and hypoxia transcriptional pathways require ARNT, some of the effects of TCDD and similar types of ligands could be explained by interaction between the AhR and hypoxia pathways involving ARNT. The studies on which we report here were conducted to test the hypothesis that there is cross talk between AhR- and HIF-1-mediated transcription pathways. TCDD significantly reduced the hypoxia-mediated reporter gene activity in B-1 cells. Reciprocally, the hypoxia response inducers desferrioxamine or CoCl(2) inhibited AhR-mediated CYP1A1 enzyme activity in B-1 and Hepa 1 cells, and the AhR-mediated luciferase reporter gene activity in H1L1.1c2 cells. The inhibition of AhR-mediated transcription by hypoxia inducers, however, was not observed in H4IIE-luc cells. The interaction between the AhR- and HIF-1-mediated transcription can be attributed to changes in DNA binding activities. TCDD-induced protein binding to dioxin responsive element (DRE) was diminished by desferrioxamine, and TCDD reduced the binding activity to HIF-1 binding site in desferrioxamine-treated Hepa 1 cells. This mutual repression may provide an underlying mechanism for many TCDD-induced toxic responses. The results reported here indicate that there is cross talk between ARNT-requiring pathways. Since ARNT is possibly required by a number of pathways, this type of interaction may explain some of the pleiotropic effects caused by TCDD.