Kinetics of catecholamine biosynthesis and metabolism have been examined in patients with hereditary progressive dystonia with marked diurnal fluctuation of symptoms (HPD, Segawa's disease). Three patients and a healthy control received an oral load of deuterated tyrosine, and monodeuterium labelled catecholamines and their metabolites in urine and plasma were examined by gas chromatography-mass spectrometry. Patients excreted normal amounts of the primary metabolites of dopamine (dihydroxyphenylacetic acid, homovanillic acid) in urine, suggesting normal rates of dopamine production. However, the biological half-life of dopamine in the patients was reduced to about half that of controls. Noradrenaline biosynthesis and metabolism were normal. Taken together, these results are interpreted to show a reduced biological half-life of dopamine in the brains of these patients, possibly caused by a defect in dopamine storage. Impaired dopamine storage may be the basis of the diurnal fluctuation in symptoms.