Glutathione transferase activity and total glutathione (GSH) content were measured in several regions of autopsied brain from patients dying with idiopathic Parkinson's disease (PD) or progressive supranuclear palsy (PSP), and from control subjects. A significant deficiency of GSH was found in the substantia nigra, but not in 5 other brain regions of PD patients, nor in PSP patients' brains. Glutathione transferase activity was similar in the substantia nigra of PD, PSP and control patients. Since total GSH is consumed only by conjugation in detoxification processes, nigral GSH deficiency in PD patients implies continued local presence of a possible causative neurotoxin up to the time of death.