Free radical-generated F₂-isoprostanes are a group of compounds with vasoconstrictor properties. To investigate whether estradiol exerts antioxidant actions modifying F₂-isoprostane production, cultured human umbilical vein endothelial cells were exposed to estradiol and other compounds and F₂-isoprostanes measured in culture medium. Exposure to 1 and 10 nM estradiol during 24 hours reduced F₂-isoprostane production by 36% and by 49%, respectively (p < 0.001 vs. control). Exposure to pure antiestrogens alone (ICI 182780 or EM-652) slightly reduced F₂-isoprostanes (p < 0.05 vs. control), but much less than estradiol (p < 0.05). ICI 182780 reversed the estradiol-induced reduction of F₂-isoprostane concentration (p < 0.05). Along with time course analysis, these results suggest that estradiol effects were mediated through both estrogen receptor-dependent and -independent mechanisms. Progestogens alone (progesterone or medroxyprogesterone acetate) did not modify F₂-isoprostanes production at any of the tested concentrations (1, 10 and 100 nM). Progesterone completely reversed estradiol-induced reduction of F₂-isoprostane production (p < 0.05 vs. control and estradiol) but medroxyprogesterone acetate did not (p < 0.05 vs. control).