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 were measured in culture medium. Exposure to 1 and 10 nM estradiol for 24 h reduced F₂-isoprostane production by 36 and 49%, respectively (P < 0.001 vs. control). Exposure to antiestrogens alone (ICI-182780 or EM-652) slightly reduced F₂-isoprostanes (P < 0.05 vs. control), but much less than exposure to 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 estrogen receptor-dependent and -independent mechanisms. Progestogens alone (progesterone or medroxyprogesterone acetate) did not modify F₂-isoprostane 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).