Estetrol is a natural compound that is produced by the fetus during fetal life and circulates in the unborn child and the mother. It is an estrogenic compound. In this study the safety and tolerability of 28 days of the oral administration of estetrol in healthy postmenopausal women are investigated. In addition, the pharmacokinetics and some pharmacodynamic parameters are studied. The lowest dose of 2 mg estetrol is directly compared with 2 mg estradiol.