There is evidence in the literature that some women experience dysphoric symptoms while receiving hormone replacement therapy (HRT) and that this disturbance in mood is related to the progestin component of the HRT. The bulk of this evidence is anecdotal. While some authors have attempted to examine this putative problem in a more systematic fashion, there are no controlled studies that attempt to identify the mechanism through which the perturbation in mood occurs. Adverse effects of progestins might be mediated directly through the progesterone or androgen receptor. Alternatively, the effects of progestins might be consequent to the antiestrogen effects of progesterone. This latter possibility is in part supported by our observation in previous studies of the beneficial effects of estradiol on mood and the possible precipitation of mood disturbance following acute estrogen withdrawal. Finally, despite the popular lore that progesterone causes mood disturbances, a placebo effect cannot be ruled out, since women taking HRT know when they are receiving the progestin component of the regimen. Our research questions therefore are as follows: 1) Are progestins associated with changes in mood during HRT, and 2) If progestins are associated with mood disturbance, is it because they are blocking the beneficial effects of estrogen?