Lifetime prevalence rates of depression are higher in women than men. Because this gender disparity appears after the onset of puberty and declines after menopause, gonadal hormones may play a role in women's increased vulnerability to dysphoric states. Estrogens have powerful effects beyond their role in reproduction. Fluctuations in estrogen occur naturally throughout the reproductive years and can be associated with disruptions in mood. Treatment for depression with exogenous estrogen has produced equivocal results. To shed light on the complex interactions among estrogens, serotonin, and mood, we briefly examine (a) central serotonin systems and their relationship to mood and mood disorders, (b) nonreproductive effects of estrogens on those systems, (c) potential points of intersection between serotonin systems and estrogens, and (d) research into the use of exogenous estrogen in depression in women. In conclusion, we reiterate the call for carefully controlled research into the etiology and treatment of depression in women.