Clinical experience and studies of heredity indicate that the etiology of the manic-depressive illness is physiologic. Examination of the possible sites of the physiopathology implicates diencephalic-rhinencephalic-reticular brain systems. It is the secondary involvement of the cerebral cortex that results in psychopathology. Psychodynamic mechanisms use the patient's experiences and reaction patterns in the formation of symptoms. As the physiopathology spontaneously improves, so do the symptoms.