Inclusion Criteria:

* A diagnosis of Major Depressive Disorder (MDD), first episode or recurrent, by Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition (DSM-IV) requiring treatment. The index episode must be at least 14 days of persistent symptoms. If first episode, patients must not have been previously treated. If recurrent, must not be receiving treatment for the recurrence.
* Females and males 20-65 years of age
* Female patients of childbearing potential must be using a reliable method of contraception and have a negative urine human chorionic gonadotropin (HCG) test at time of enrolment
* Able to understand and comply with the requirements of the study

Exclusion Criteria:

* Females who are pregnant, lactating, breast feeding or on oral contraceptives
* Any DSM-IV Axis I disorder not defined in the inclusion criteria except MDD co-morbid with generalized anxiety disorder (GAD)
* Patients who, in the opinion of the investigator, pose an imminent risk of suicide or a danger to self or others
* Known intolerance or lack of response to quetiapine (Seroquel) as judged by the investigator
* Use of any of the following cytochrome P450 3A4 inhibitors in the 14 days preceding enrolment including but not limited to: ketoconazole, itraconazole, fluconazole, erythromycin, clarithromycin, troleandomycin, indinavir, nelfinavir, ritonavir, fluvoxamine and saquinavir
* Use of any of the following cytochrome P450 inducers in the 14 days preceding enrolment including but not limited to: phenytoin, carbamazepine, barbiturates, rifampin, St. John's Wort, and glucocorticoids
* Concomitant use of any other antidepressant, anxiolytic, or antipsychotic agent
* Administration of a depot antipsychotic injection within one dosing interval (for the depot) before the study begins
* Substance or alcohol dependence at enrolment (except dependence in full remission, and except for caffeine or nicotine dependence), as defined by DSM-IV criteria
* History of heavy smoking within the preceding 6 months
* Opiates, amphetamine, barbiturate, cocaine, cannabis, or hallucinogen abuse by DSM-IV criteria within 4 weeks prior to enrolment
* Restrictions prior to blood drawings: Aspirin (previous 240 hours), antihistamines (previous 72 hours), Tylenol (previous 72 hours), Vitamin C or E (previous 72 hours), sleeping pills (previous 72 hours), caffeinated beverages (8 hours), physical exertion (8 hours) and tobacco products (2 hours).
* Medical conditions that would affect absorption, distribution, metabolism, or excretion of study treatment
* Unstable or inadequately treated medical illness (e.g. diabetes, angina pectoris, hypertension) as judged by the investigator
* Involvement in the planning and conduct of the study
* Previous enrolment in the present study.
* Participation in another drug trial within 4 weeks prior enrolment into this study or longer in accordance with local requirements
* A patient with Diabetes Mellitus (DM) fulfilling one of the following criteria:

* Unstable DM defined as enrolment glycosylated hemoglobin (HbA1c) \>8.5%.
* Admitted to hospital for treatment of DM or DM related illness in past 12 weeks.
* Not under physician care for DM
* Physician responsible for patient's DM care has not indicated that patient's DM is controlled.
* Physician responsible for patient's DM care has not approved patient's participation in the study