The definition of estrogen's actions has expanded from transcriptional regulation to the rapid, membrane-initiated activation of numerous signal transduction cascades. Multiple biological effects of estrogen have been shown in numerous animals, cellular and molecular studies, which support the favorable effects of estrogen on vascular structure, function, and cell signaling. Work from several laboratories has shown that these effects are mediated by distinct forms of estrogen receptor (ER) alpha. This includes estrogen-stimulated rapid activation of endothelial nitric oxide synthase (eNOS), resulting in the elaboration of the athero-protective, angiogenesis-promoting product nitric oxide (NO). We have described the expression of ER46, an N-terminus truncated isoform of the ERalpha, in human endothelial cells (EC), and its critical role in membrane-initiated, rapid responses to 17beta-estradiol (E2). We have proposed an ER46-centered, eNOS activating molecular complex in human EC caveolar membranes, containing c-Src, phosphatidylinositol 3-kinase (PI3K), Akt and eNOS. Our previous studies support estrogen-induced rapid eNOS activation via a sequential c-Src/PI3K/Akt cascade in EC. In this review, we describe estrogen-induced, rapid, non-genomic actions in endothelium, driven by c-Src-ER46-caveolin-1 interactions, with consequent activation of eNOS. Amidst ongoing controversies in hormone replacement therapy, these molecular and cellular data, defining favorable estrogenic effects on the endothelium, provide a strong impetus to resolve these clinical questions.