An alteration in the brain serotonergic system in individuals with depression is proposed based on reduced 5-hydroxyindoleacetic acid levels in cerebrospinal fluid. Some studies of suicides also show reduced brain serotonin concentrations. Other investigations of serotonin-related functions in depressed patients have yielded equivocal results. Additional information is needed to further understanding of serotonergic system alterations relative to affective disorders. 147 refs.