The purpose of this study is to determine the effect of a single dose infusion of AZD 6765 compared with placebo and ketamine on the blood oxygen level dependent signal using fMRI (functional Magnetic Resonance Imaging) in depressed subjects.

Inclusion Criteria:

• Documented clinical diagnosis meeting criteria from the DSM-IV by structured clinical interview,major depressive disorder, single episode or recurrent
• Outpatient status at screening and at randomisation

Exclusion Criteria:

• A major depression disorder which has a major impact on the subjects current psychiatric status
• Subjects not allowed to use mood stabilizers, antidepressants or other antipsychotic or psychoactive drugs
• Subjects with lifetime history of schizophrenia, bi-polar, psychosis and psychotic depression