Major depressive disorder (MDD) has been associated with reduced functioning of central serotonergic systems. Tryptophan depletion (TD) is a procedure used to investigate the relationship between serotonergic function and depression. Evidence suggests that the mood lowering effects of TD depend upon family history and differences in genes for a specific protein called 5-HTTLPR. Healthy females with a particular gene for 5-HTTLPR and a family history of mood disorders appear to be at a greater risk for the development of depressive symptoms during TD. This study will use positron emission tomography (PET) scans of the brain to investigate the effect of variant 5-HTTLPR genotypes on response to TD. The relationship between 5-HTTLPR genotypes and the effect of TD on brain activity in individuals with different 5-HTTLPR genes will be determined. This study will also examine how the reduced serotonin function that occurs in MDD affects the brain's response to sensory stimulation.