Treatment of post‐menopausal women with ethinyloestradiol before operation for prolapse in a dose of 250 micrograms a day for 10 days significantly suppressed the fibrinolytic activity–determined histochemically–in the vein walls. It enhanced the spontaneous fibrinolytic activity in the blood, and the response of the local fibrinolytic activity of the blood to venous occlusion of the arms tended to increase. These findings may be explained by the assumption that ethinyloestradiol inhibits the synthesis of activators of fibrinolysis or stimulates their release to a rate exceeding that of their production. Each alternative would result in diminution of the activator content of the vessel walls.