The pathologic changes of parkinsonism and the side effects of drug therapy may produce pronounced behavioral changes in patients with Parkinson's disease. Dementia is the primary manifestation of disease-induced mental alterations and careful pharmacologic management is necessary. Psychosis is the most dramatic of the changes induced by levodopa. It may occur early in the course of therapy, usually in patients with a past history of a schizophreniform disorder, or after several years of treatment. Therapeutic intervention with dosage adjustments and/or drug holidays are indicated when psychosis occurs. Treatment of depression in the Parkinson patient is essentially the same as in a nonparkinson patient.