The authors investigated the melatonin response to 5-methoxypsoralen in 39 healthy adult and elderly subjects and in 13 demented, 13 depressed, and 13 schizophrenic inpatients. Subjects' plasma melatonin levels were evaluated before they took single, oral 40-mg doses of 5-methoxypsoralen and 3 hours afterward. The preliminary results indicate that the melatonin response of the pineal gland to 5-methoxypsoralen is reduced in demented patients compared to that in healthy elderly subjects. Depressed and schizophrenic patients had a normal melatonin response to the drug. This finding suggests that the pineal gland may be functionally impaired in dementia but not in depression.