The theory that abnormalities in the physiology and metabolism of biogenic amines—particularly the catecholamines dopamine and norepinephrine and the indoleamine serotonin—are involved in the causes and courses of certain psychiatric illnesses. This hypothesis was derived originally from a serendipitous discovery that monoamine oxidase inhibitors (maois) and certain tricyclic drugs had mood-elevating properties and that these agents exerted dramatic effects on brain monoamine functions. The finding that phenothiazines and other conventional antipsychotics inhibit dopamine activity in the brain further supports this theory and suggests that a disorder of dopamine metabolism may be implicated in the etiology of psychosis or mania. Also, disorders in norepinephrine and serotonin activity have been implicated in the etiology of depression and mania.