Previous studies have demonstrated that 2,3,7,8 tetracholorodibenzo-p-dioxin (TCDD) mimics epidermal growth factor receptor (EGFR) signaling in the MCF-10A human mammary epithelial cell line and protects cells from EGF withdrawal-induced apoptosis. These effects appear to be due to the ability of TCDD to increase the expression of transforming growth factor-alpha (TGFalpha), a known EGFR ligand. Because TCDD's effects occurred at concentrations as low as 1 nM, a role for the aryl hydrocarbon receptor (AhR) was hypothesized. In the present study, 3'methoxy-4'nitroflavone (MNF), a known AhR antagonist, was used to analyze AhR signaling in this cell line. MNF suppressed TCDD-dependent dioxin response element (DRE)-driven luciferase activity at concentrations as low as 10 nM. In addition, MNF attenuated TCDD's ability to inhibit apoptosis and to activate Akt and Erk1,2, two EGFR-dependent signaling molecules. Finally, the TCDD-dependent increase in TGFalpha mRNA was also suppressed by MNF. MNF's effects on TCDD action in the MCF-10A cell line occurred at concentrations ranging from 1 nM for Akt phosphorylation and TGFalpha expression to 100 nM for inhibition of apoptosis. Attenuation of TCDD-dependent luciferase activity occurred at concentrations as low as 10 nM, which suggests that TCDD inhibits apoptosis in human mammary epithelial cells by multiple mechanisms.