2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds induce a broad spectrum of biochemical and toxic responses and disrupt multiple endocrine pathways. Research in this laboratory has focused on characterizing aryl hydrocarbon receptor (AhR)-mediated antiestrogenicity in the rodent uterus and mammary and in human breast cancer cells. TCDD inhibits multiple estrogen (E2)-induced responses in these tissues including development or growth of human mammary and endometrial cancer cells, carcinogen-induced mammary cancer in rats, and mammary cancer in mice bearing breast cancer cell xenografts. The mechanisms of AhR-mediated antiestrogenicity are complex; however, studies on the molecular biology of cross-talk between the AhR and estrogen-receptor (ER) signaling pathways have been initiated using several E2-regulated genes as models. The results indicate that the nuclear AhR complex targets specific genomic core inhibitory dioxin responsive elements (iDREs) in promoter regions of some E2-responsive target genes to inhibit hormone-induced transactivation. The pS2, cathepsin and c-fos genes have functional iDREs, whereas the iDRE in the progesterone receptor gene promoter was not functional. Research has also focused on development of AhR-based antiestrogens which inhibit mammary tumor development and growth but do not exhibit prototypical AhR-induced toxic responses.