The study investigates the interaction between the estrogen receptor (ER) and the regulatory subunit of phosphatidylinositol-3-OH kinase (PI(3)K) to elucidate a non-nuclear signaling pathway of estrogen. The authors demonstrate that ERα binds ligand-dependent to the p85α subunit of PI(3)K, which, upon estrogen stimulation, increases PI(3)K activity and activates protein kinase B/Akt and endothelial nitric oxide synthase (eNOS). This interaction is independent of gene transcription and does not involve phosphotyrosine adapter molecules or src-homology domains of p85α. The vascular protective effects of estrogen, such as increased eNOS activity and reduced vascular leukocyte accumulation after ischemia and reperfusion injury, are abolished by PI(3)K or eNOS inhibitors. The findings suggest a novel, physiologically important non-nuclear estrogen signaling pathway involving direct interaction between ERα and PI(3)K.The study investigates the interaction between the estrogen receptor (ER) and the regulatory subunit of phosphatidylinositol-3-OH kinase (PI(3)K) to elucidate a non-nuclear signaling pathway of estrogen. The authors demonstrate that ERα binds ligand-dependent to the p85α subunit of PI(3)K, which, upon estrogen stimulation, increases PI(3)K activity and activates protein kinase B/Akt and endothelial nitric oxide synthase (eNOS). This interaction is independent of gene transcription and does not involve phosphotyrosine adapter molecules or src-homology domains of p85α. The vascular protective effects of estrogen, such as increased eNOS activity and reduced vascular leukocyte accumulation after ischemia and reperfusion injury, are abolished by PI(3)K or eNOS inhibitors. The findings suggest a novel, physiologically important non-nuclear estrogen signaling pathway involving direct interaction between ERα and PI(3)K.