Estradiol can stimulate prostacyclin production in the vessel wall, thereby eliciting vasodilatation. In the present work the effect of the estradiol metabolites estrone, 2-methoxyestrone, 2-methoxyestradiol, and 16alpha-hydroxyestrone were investigated to find out if they are also able to stimulate prostacyclin synthesis. All metabolites triggered an increase of prostacyclin synthesis in human endothelial cells starting at a concentration of 10(-9) M. The parent substance, 17beta-estradiol, accomplished this effect only starting at a concentration of 10(-8) M. These results indicate that estradiol metabolites may take part in the estradiol-induced vasodilatation in vivo.