Estradiol retards the development of atherosclerosis. Animal models have suggested that NO may be a critical effector molecule in this cardiovascular protection. In this study, female human umbilical vein endothelial cells (HUVECs) were propagated in phenol red-free gonadal hormone-free medium and pretreated with 17 beta-estradiol (E2). Reduced NO2- and NO3- (NOx) concentration, determined by chemiluminescence, demonstrated a rapid increase in basal HUVEC NO release in response to physiological concentrations of E2. The estrogen receptor (ER) antagonist ICI 164,384 inhibited the augmented NO release, demonstrating an ER-mediated component of this response. Because endothelial NO synthase (eNOS) activity is largely regulated by cytosolic Ca2+, relative [Ca2+]i in response to E2 was determined in a fluorometric assay. E2 did not promote HUVEC Ca2+ fluxes. Furthermore, eNOS activity in E2-pretreated endothelial whole-cell lysates was not dependent on additional Ca2+. Despite involving the ER, this is a nongenomic effect E2, as demonstrated by maintained responses in transcriptionally inhibited cells and by the rapidly (10 minutes) of cGMP formation in an NO bioassay. We demonstrate, for the first time, that independent of cytosolic Ca2+ mobilization, there is augmentation of eNOS activity with a resultant increase in HUVEC basal NO release in response to short-term estradiol exposure. Implications for the cardiovascular protective role of estrogen are discussed.