Clinical and experimental data support the consideration of endothelium as a target for estradiol and other estrogenic compounds. Estradiol acts in the endothelium to promote vasodilatation through release of several compounds, including synthesis of prostanoids, products of arachidonic acid metabolism. Two main prostanoids play an essential role in vascular physiology: thromboxane A2 (TXA2), which exhibits a proaggregant and vasoconstrictor profile, and prostacyclin (PGI2), a potent vasodilator. The different role of both types of estrogen receptors (ERα and ERβ) on the balance PGI2/TXA2 has not been studied. Our aim were to uncover whether estradiol enhance basal production of PGI2 or TXA2 in cultured human umbilical vein endothelial cells (HUVEC), to analyze the enzymatic mechanisms involved and to value the different role of both types of ER. HUVEC were exposed to estradiol, selective ERα (PPT) or ERβ (DPN) agonists and antagonists (inespecific: ICI182780; specific for ERα: MPP) for 24 hours. PGI2 and TXA2 production was measured by ELISA. Production and expression of phopholipases, cyclooxygenases (COX-1 and COX-2), prostacyclin synthase and thromboxane synthase were analyzed by Western blot and quantitative RT-PCR. Estradiol (1-100 nM) dose-dependent increased PGI2 production, up to 50 % (p < 0.001 vs. control), without affecting TXA2 production. COX-1 and prostacyclin synthase protein and gene expression were increased by 20 and 50 % respectively after exposure to estradiol (p < 0.001 vs. control), whereas COX-2, phospholipases and thromboxane synthase expression remained unaltered. All these effects were meditated through ERα, since were produced not only in the presence of estradiol but also in the presence of 10 nM PPT, and completely abolished in the presence of 1 µM MPP. In conclusion, estradiol, acting through ERα, up-regulates COX-1 and prostacyclin synthase expression, thus directing prostanoid balance towards increased PGI2 production.