Adults with current Major Depressive Disorder or Bipolar Depression

Inclusion Criteria:

1. Provision of written informed consent;
2. A diagnosis of Bipolar Disorder Type 1,2 or NOS by Diagnostic and Statistical Manual of Mental Disorders- Fourth Edition (DSM-IV);OR Major Depressive Disorder
3. Males or Females aged 18 years or more;
4. Female patients of childbearing potential must be using a reliable method of contraception and have a negative urine human chorionic gonadotropin (HCG) test at enrolment;
5. Able to understand and comply with the requirements of the study;
6. Current depressive episode with a HAM-D17 score of 15 or more.

Exclusion Criteria:

1. Current Manic, Hypomanic or Mixed episode, with YMRS 12 or more;
2. Current or past diagnosis of Schizophrenia;
3. Pregnant women, or women in childbearing age, not willing to use appropriate contraception or women currently nursing;
4. Patient on antipsychotic medication;
5. Patients who, in the opinion of the investigator, pose an imminent risk of suicide or a danger to self or others;
6. Known intolerance or lack of response to quetiapine fumarate, as judged by the investigator;
7. 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;
8. 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;
9. Administration of a depot antipsychotic injection within one dosing interval (for the depot) before randomization;
10. Substance or alcohol dependence at enrolment (except dependence in full remission, and except for caffeine or nicotine dependence), as defined by DSM-IV criteria;
11. Opiates, amphetamine, barbiturate, cocaine, cannabis, or hallucinogen dependence by DSM-IV criteria within 4 weeks prior to enrolment;
12. Medical conditions that would affect absorption, distribution, metabolism, or excretion of study treatment;
13. Unstable or inadequately treated medical illness (e.g., diabetes, angina pectoris, hypertension) as judged by the investigator;
14. Involvement in the planning and conduct of the study;
15. Previous enrolment in the present study;
16. Participation in another drug trial within 4 weeks prior enrolment into this study or longer in accordance with local requirements;
17. If participant's liver function testing is rated 2 in the upper limits of normal.
18. Diagnosis of Dementia
19. Regular use of benzodiazepines unless at a stable dose for at least 12 weeks.