 Subjects must not have taken antidepressant or other medications likely to alter monoamine neurochemistry or cerebrovascular function for at least 3 weeks 8 weeks for fluoxetine and for any drug with known anticholinergic effects prior to scanning. Because effective medications will not be discontinued for the purposes of this study, subjects will be identified who have never been treated or who have discontinued medication due to lack of efficacy, noncompliance, physician order or other reasons prior to study entry. Subjects will be excluded if they: a have had serious suicidal ideation or behavior in the previous two months, or b psychosis, or c medical or neurological illnesses i.e. seizure disorder, a coma in past likely to affect physiology or anatomy, d a history of drug or alcohol abuse within 1 year or a lifetime history of alcohol or drug dependence DSM IV criteria, e are HIV positive or have AIDS, fcurrent pregnancy documented by history and pregnancy testing prior to scanning, g current breast feeding, h general MRI exclusion criteria which include the subject having a pacemaker or significant claustrophobia. 