64 Caucasians of both gender with an age range of 20-55 years meeting the criteria of DSM-IV (American Psychiatric Association 2000) criteria for major depressive disorder according to the Structured Clinical Interview (First et al.,2000) for DSM-IV Axis 1 disorders will be recruited from the Calgary Health Region outpatient mental health clinics and through local newspaper advertisements. Eligible patients will be randomly assigned in a 1:1 ratio to receive either quetiapine or citalopram. Thus each treatment arm will have 32 subjects.