The current study employs quantitative PET imaging of cerebral blood flow (CBF) and glucose metabolism to investigate the effect of variant 5-HTTLPR genotypes on the neurophysiological response to TD. The current study will examine the relationship between 5-HTTLPR genotypes and the TD effect on PFC metabolic activity. We will determine whether under TD the reduction in PFC metabolism in response to TD will occur to a greater extent in subjects with the s/s allele, and in subjects with a single s allele plus a family history of depression. We will also examine whether this reduction in PFC metabolic activity is unique to subjects who develop depressive symptoms during TD.