This study investigated the effects that 4-nonylphenol (NP) has on CYP1A1 expression in Hepa-1c1c7 cell cultures. NP alone did not affect CYP1A1-specific 7-ethoxyresorufin-O-deethylase (EROD) activity. In contrast, the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible EROD activities were markedly reduced upon concomitant treatment with TCDD and NP in a dose-dependent manner. Treatment with tamoxifen, an anti-estrogen that acts through the estrogen receptor, did not affect the suppressive effects that NP has on TCDD-inducible EROD activity. The TCDD-inducible CYP1A1 mRNA levels were markedly suppressed upon concomitant treatment with TCDD and NP that is consistent with their effects on EROD activity. A transient transfection assay using dioxin-response element (DRE)-linked luciferase and an electrophoretic mobility shift assay revealed that NP reduced the transformation of the aryl hydrocarbon (Ah) receptor to a form capable of binding specifically to the DRE sequence of the CYP1A1 gene promoter. These results suggest that the down-regulation of CYP1A1 gene expression by NP in Hepa-1c1c7 cells might be an antagonism of the DRE-binding potential of the nuclear Ah receptor, but is not mediated through the estradiol receptor.