Menopausal and postmenopausal women compose almost 20% of the Austrian population. Two thirds of all austrian women suffering from depression or anxiety disorders are over 45 years old. The serotonergic system, partially regulated by the steroid hormones estrogen and progesterone, plays a major role in the pathogenesis and treatment of these illnesses. To examine the effect of the hormone replacement therapy on the serotonergic system, twenty-four postmenopausal women will be measured using positron emission tomography (PET). The volunteers will participate in two PET scans. The first PET scan will be performed right before the hormone treatment starts, the second PET scan about 8 weeks after daily treatment with (1) a combination of estrogen and progesterone or (2) estrogen and placebo. This imaging study hypothesizes that the expression of the main inhibiting serotonergic receptor (the serotonin-1A receptor) will be altered by the hormone therapy. The results of the study might lead to new strategies in the treatment of psychiatric illnesses during and after the menopausal transition.