Estriol (E3) and estradiol (E2) was given to 81 women with perimenopausal complaints. Venous blood was sampled before and after 2 weeks’ treatment. In plasma coagulation and fibrinolysis the most marked estrogen effects were seen within the extrinsic coagulation system, with increased Thrombotest and Normotest activities. The incidence of cold activation of factor VII showed a marked increase after treatment with 6 mg of E2 or 8 mg of E3. The cold activation of factor VII was correlated to an activation of the plasma kallikrein system, as revealed by the peptidase activity of plasma kallikrein. The treatment with E2 and E3 induced changes also in the plasma proteolytic capacity, and in some proteinase inhibitors (antithrombin III, C1 inactivator). Like the effects within the plasma coagulation and kallikrein systems, these effects were similar to those reported after treatment with ethinyl estradiol, mestranol, and diethylstilbestrol, and in pregnancy. The conclusion is drawn, therefore, that the coagulation and fibrinolytic effects of E2 and E3 are basically similar to those of other estrogens, the differences between the various estrogens being more of a quantitative than of a qualitative nature. For the changes described, no absolute dose dependency could be found. Thus, treatment with E2 and E3 should be submitted to the same precautions as treatment with other estrogens.