The author reviews the evidence which supports the role of serotonin (5-HT) in depression. He discussed the effect of 5-HT on mood, affect & somatic functions, as well as discussing CSF 5-Hydroxyindoleacetic acid, plasma tryptophan, blood platelet 5-HT mechanisms, neuroendocrine challenge studies, 5-HT uptake inhibitors, & serotonin & suicide. He concludes that 5-HT may be more important as a vulnerability factor in major depression than as a proximate cause. (VM)