Objectives: Biological approaches to depression have been dominated by the serotonin theory, which suggests that clinical depression is associated with low serotonin function in the brain. However, the way in which serotonin might lead to mood change is still unclear. The relationship between serotonin dysfunction and psychological mechanisms identified by cognitive theories as being fundamental in the development of depression (e.g. core beliefs, cognitive distortions, negative automatic thoughts,) is particularly unclean The low dose tryptophan paradigm will be used to examine whether or how impaired serotonin function produces cognitive changes that map directly onto the clinical cognitive constructs that are targeted in CBT for depression.