Inclusion Criteria:

• Current age ≥18 years
• DSM-IV diagnosis of BPD, type-I, or type-II
• Current major depressive episode using DSM-IV criteria, lasting 8 weeks or longer.
• Use of lithium, divalproex, carbamazepine, or lamotrigine at therapeutic serum levels or doses for ≥4 weeks prior to study entry, or willingness to accept one of these agents.
• Prior to initial evaluations, each subject must provide competent, written, informed consent.

Exclusion Criteria:

• Past non-response to a therapeutic trial of R,S-citalopram (≥100 mg/day for ≥8 weeks).
• Previous intolerance of R,S-citalopram;
• Diagnosis of unipolar depression
• Diagnosis of schizoaffective disorder
• Serious medical illness with acute instability (cardiac, respiratory, hepatic, renal), based on hospitalization in the past month
• Abnormal thyroid function tests
• Previous allergic reaction to or inability to tolerate lithium, divalproex, or carbamazepine at therapeutic serum levels.
• Current or past renal dysfunction if taking lithium
• Current or past hepatitis or other liver disease if taking divalproex
• Current or past hematologic disease if on carbamazepine
• Severe suicidal ideation, plan or intent, as documented by a score of ≥4 on the Montgomery Åsberg Depression Rating Scale suicidality item (Item 10).
• Presence of psychosis
• Cognitive impairment sufficient to impair ability to give informed consent.
• Current pregnancy, or inability to utilize contraception
• The presence of any metallic implants
• History of claustrophobia