 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 also be excluded if they: a serious suidical ideation or behavior - 1 thoughts of suicide within the past three months which are accompanied by intet to harm oneself, serious consideration of means or plan to attempt suicide, evidence of arranging for a suicide attemp e.g. giving away prized possessions, updating a will or clear desire to commit suicide; 2 suicide attempts within the previous one year; or 3 a current plan to inflict self harm or physical evidence suspicious for having engaged in a suicide attempt. 