Glutathione peroxidase is an enzyme of major importance in the detoxification of peroxides in brain. Using the spectrophotometric procedure of Paglia and Valentine [8] and Beutler [2] we measured the activity of this enzyme in autopsied brain from 12 patients dying with idiopathic Parkinson's disease and 11 neurologically normal adults matched with respect to age and postmortem interval. In the Parkinson's disease patients glutathione peroxidase activity was slightly but significantly reduced in several brain areas including substantia nigra. Although the magnitude of the glutathione peroxidase deficiency in Parkinson's disease substantia nigra was small (19% reduction), coupled with the reported marked deficiency of reduced glutathione [9] it may represent one of the contributing factors leading to nigral dopamine neurone loss.