Subjects will be screened with an on-site research psychiatric evaluation using a structured interview (MINI plus). After screening, participants will provide a DNA sample, which will be analyzed to determine the patient s genotype at several relevant loci. All patients will then complete an eight-week period of citalopram treatment, during which time they will be evaluated bi-weekly with self-report questionnaires and clinician ratings to determine change of MDD symptoms. Participants will also undergo pre- and post-treatment MRS scans to determine glutamate concentration in the prefrontal brain.