Subjects will be recruited who are drug-na ve or who have not received psychotropic drugs for at least 3 weeks (8 weeks for fluoxetine) prior to scanning. Effective medications will not be discontinued for the purposes of the study. Subjects will also be excluded if they have a) serious suicidal ideation or behavior, b) psychosis to the extent that the ability to provide informed consent is in doubt, c) medical conditions or concomitant medications (see Appendix III) that are likely to influence cerebral blood flow or neurological function including cardiovascular, respiratory, endocrine and neurological diseases, d) a history of drug or alcohol abuse within 1 year or a lifetime history of alcohol or drug dependence (DSM-IV criteria), e) current pregnancy (as documented by pregnancy testing prior to scanning), f) general MRI exclusion criteria, g) history of non-response to Zoloft or of intolerable or adverse side effects during Zoloft treatment. Additional exclusion criteria applied to control subjects are: a) subjects with a current or past history of other axis I psychiatric conditions, b) subjects with first-degree family members with current or past history of mood disorder.