It is shown that the platelet serotonin level and the maximum rate of uptake of labeled serotonin by platelets in the groups of patients with anxious-depressive and depressive-hysterical syndromes is markedly higher than in the control (patients with psychopathy without signs of depression) and in the group with the asthenodepressive syndrome. In all groups of patients with depressions the excretion of 5-hydroxyindoleacetic acid is reduced as compared to the control.