Reduced platelet MAO activity has been previously reported as a biochemical marker for a subgroup of psychiatric patients, including some chronic schizophrenics. As tryptamine metabolism appears to be sensitive to alterations in MAO activity, urinary tryptamine excretion was measured in chronic schizophrenics with low platelet MAO activity, chronic schizophrenics with normal platelet MAO activity, and age-matched and sex-matched controls. The increased urinary tryptamine excretion observed in chronic schizophrenics with low platelet MAO activity may reflect a pathophysiologic mechanism associated with low MAO activity.