Estradiol (E(2)) acts on the endothelium to promote vasodilatation through the release of several compounds, including prostanoids, which are products of arachidonic acid metabolism. Among these, prostacyclin (PGI2) and thromboxane A2 (TXA2) exert opposite effects on vascular tone. The role of different estrogen receptors (ERs) in the PGI2/TXA2 balance, however, has not been fully elucidated. Our study sought to uncover whether E(2) enhances basal production of PGI2 or TXA2 in cultured human umbilical vein endothelial cells (HUVECs), to analyze the enzymatic mechanisms involved, and to evaluate the different roles of both types of ERs (ERalpha and ERbeta). HUVECs were exposed to E(2), selective ERalpha (1,3,5-tris(4-hydroxyphenyl)-4-propyl-1h-pyrazole, PPT) or ERbeta (diarylpropionitrile, DPN) agonists and antagonists (unspecific: ICI 182 780; specific for ERalpha: methyl-piperidino-pyrazole, MPP). PGI2 and TXA2 production was measured by ELISA. Expression of phospholipases, cyclooxygenases (COX-1 and COX-2), PGI2 synthase (PGIS), and thromboxane synthase (TXAS) was analyzed by western blot and quantitative RT-PCR. E(2) (1-100 nM) dose dependently increased PGI2 production (up to 50%), without affecting TXA2 production. COX-1 and PGIS protein and gene expressions were increased, whereas COX-2, phospholipases, and TXAS expression remained unaltered. All these effects were mediated through ERalpha, since they were produced not only in the presence of E(2), but also in that of PPT, while they were abolished in the presence of MPP. In conclusion, E(2), acting through ERalpha, up-regulates COX-1 and PGIS expression, thus directing prostanoid balance toward increased PGI2 production.