Estrogen-responsive breast cancer cells, such as MCF7 and T47D cells, express both estrogen receptor (ER)-alpha (ERalpha) and ERbeta. Indole-3-carbinol (I3C) strongly down-regulated ERalpha protein and transcript levels, without altering the level of ERbeta protein, in both cell lines. In cells transfected with the ERalpha promoter linked to a luciferase gene reporter, I3C ablated ERalpha promoter activity. Propyl pyrazole triol (PPT) is a highly selective ERalpha agonist, whereas, 17beta-estradiol activates both ERalpha and ERbeta. I3C treatment inhibited the PPT- and 17beta-estradiol-induced proliferation of breast cancer cells, disrupted the PPT and 17beta-estradiol stimulation of estrogen response element (ERE)-driven reporter plasmid activity as well as of endogenous progesterone receptor transcripts. Using an in vitro ERE binding assay, I3C was shown to inhibit the level of functional ERalpha and stimulated the level of ERE binding ERbeta even though the protein levels of this receptor remained constant. In ERalpha-/ERbeta+ MDA-MB-231 breast cancer cells, I3C treatment stimulated a 6-fold increase in binding of ERbeta to the ERE. I3C also induced ERE- and activator protein 1-driven reporter plasmid activities in the absence of an ER agonist, suggesting that ERbeta is activated in indole-treated cells. Taken together, our results demonstrate that the expression and function of ERalpha and ERbeta can be uncoupled by I3C with a key cellular consequence being a significantly higher ERbeta:ERalpha ratio that is generally highly associated with antiproliferative status of human breast cancer cells.