The antitumorigenic activities of 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF), 8-methyl-1,3,6-trichlorodibenzofuran (8-MCDF) and 6-cyclohexyl-1,3,8-trichlorodibenzofuran (6-CHDF) were investigated in the 7,12-dimethylbenz[a]anthracene (DMBA) rat mammary tumor model. At doses of 5, 10 or 25 mg/kg/week, both 6-MCDF and 8-MCDF significantly inhibited mammary tumor growth and at the 5 mg/kg/week dose >50% growth inhibition was observed. In contrast, 6-CHDF was inactive at the 5 mg/kg/week dose and the structure-antitumorigenicity relationships (6-MCDF/8-MCDF > 6-CHDF) correlated with structure-antiestrogenicity (rat uterus) studies and the relative binding affinities of these compounds for the aryl hydrocarbon receptor (AhR). The antitumorigenic activity of 6-MCDF or 8-MCDF in the mammary was not accompanied by any significant changes in liver/body weight ratios, liver morphology or induction of hepatic CYP1A1-dependent activity which is one of the most sensitive indicators of exposure to AhR agonists. RT-PCR and Western blot analysis of mammary tumor mRNA and protein extracts, respectively, confirmed the presence of AhR suggesting that AhR-mediated signaling pathways are functional in rat mammary tumors. These results define a relatively non-toxic group of AhR agonists which exhibit potent antitumorigenic activity in the DMBA-induced rat mammary tumor model (<1 mg/kg/day), and therefore represent a new class of indirect-acting antiestrogens which have potential for clinical treatment of mammary cancer.