This is an eight-week, parallel-group, randomized, double blind comparative trial with a repeated measure design in which the patients with major depression will be randomly assigned in a 1:1 ratio to receive treatment with either quetiapine 300 mg or citalopram 20mg for an 8-week period. The subjects will undergo imaging during three separate 60-minute sessions: 1) baseline or week 0, 2) one week after initiation of treatment, and 3) 8 weeks after baseline or initiation of treatment. Genotyping will be performed in all participants.