A growing body of evidence concerning estrogen effects cannot be explained by the classic model of hormone action, which involves the binding to estrogen receptors (ERs) alpha and ERbeta and the interaction of the steroid-receptor complex with specific DNA sequences associated with target genes. Using c-fos proto-oncogene expression as an early molecular sensor of estrogen action in ERalpha-positive MCF7 and ER-negative SKBR3 breast cancer cells, we have discovered that 17beta-estradiol (E2), and the two major phytoestrogens, genistein and quercetin, stimulate c-fos expression through ERalpha as well as through an ER-independent manner via the G protein-coupled receptor homologue GPR30. The c-fos response is repressed in GPR30-expressing SKBR3 cells transfected with an antisense oligonucleotide against GPR30 and reconstituted in GPR30-deficient MDA-MB 231 and BT-20 breast cancer cells transfected with a GPR30 expression vector. GPR30-dependent activation of ERK1/2 by E2 and phytoestrogens occurs via a Gbetagamma-associated pertussis toxin-sensitive pathway that requires both Src-related and EGF receptor tyrosine kinase activities. The ability of E2 and phytoestrogens to regulate the expression of growth-related genes such as c-fos even in the absence of ER has interesting implications for understanding breast cancer progression.