GPR30 is a transmembrane G protein-coupled receptor (GPCR) localized to endoplasmic reticulum (ER) that binds estradiol with high affinity, activating multiple intracellular signaling pathways.G-1 is a nonsteroidal, high-affinity, selective agonist of GPR30 that binds with a Ki value of 11 nM. Competitive binding studies in endoplasmic reticulum α- (ERα-) and ERβ-expressing cells yielded Ki values for estradiol of 0.30 and 0.38 nM, respectively, with no substantial binding of G-1 at 1 µM.The discovery of G-1, a compound that does not bind classical ERs, should facilitate further physiological experiments to define the role of GPR30 in vivo.