Accumulating evidence suggested that an orphan G protein-coupled receptor (GPR)30, mediates nongenomic responses to estrogen. The present study was performed to investigate the molecular mechanisms underlying GPR30 function. We found that knockdown of GPR30 expression in breast cancer SK-BR-3 cells down-regulated the expression levels of estrogen receptor (ER)-α36, a variant of ER-α. Introduction of a GPR30 expression vector into GPR30 nonexpressing cells induced endogenous ER-α36 expression, and cotransfection assay demonstrated that GPR30 activated the promoter activity of ER-α36 via an activator protein 1 binding site. Both 17β-estradiol (E2) and G1, a compound reported to be a selective GPR30 agonist, increased the phosphorylation levels of the MAPK/ERK1/2 in SK-BR-3 cells, which could be blocked by an anti-ER-α36-specific antibody against its ligand-binding domain. G1 induced activities mediated by ER-α36, such as transcription activation activity of a VP16-ER-α36 fusion protein and activation of the MAPK/ERK1/2 in ER-α36-expressing cells. ER-α36-expressing cells, but not the nonexpressing cells, displayed high-affinity, specific E2 and G1 binding, and E2- and G1-induced intracellular Ca²⁺ mobilization only in ER-α36 expressing cells. Taken together, our results demonstrated that previously reported activities of GPR30 in response to estrogen were through its ability to induce ER-α36 expression. The selective G protein-coupled receptor (GPR)30 agonist G1 actually interacts with ER-α36. Thus, the ER-α variant ER-α36, not GPR30, is involved in nongenomic estrogen signaling.