AbstractThis review attempts to assess critically the evidence which implicates a disorder of CNS neurotransmission in the aetiology of depression. Evidence for such a disorder has been obtained from changes in neuroendocrine function and in the activity of neurotransmitter receptors located on platelets and lymphocytes of depressed patients. Such studies are important not only because they give an insight into the possible biochemical basis of the illness, but also because they may be used as markers for the onset of depression and of response to treatment. The review concludes with a brief assessment of the change in immune responsiveness which may be associated with the onset of depression. Such studies support the view that the biological changes occurring in depression are not restricted to the brain and therefore open up the possibility that biochemical tests based on analysis of a blood sample may form a useful adjunct to clinical assessment.