Oxygen-derived toxicity has been suggested as being involved in the pathogenesis of Parkinson's disease. Superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase provide the enzymatic defence against oxygen toxicity. The activities of these enzymes were measured in peripheral blood leucocytes, cerebrospinal fluid and in different brain regions from patients with idiopathic Parkinson's disease and from controls. There was no indication of a generalized defect in any of these enzymes in Parkinson's disease. The brain activities of catalase, glutathione peroxidase and glutathione reductase were also comparable to those of the controls. An increased superoxide dismutase-like activity was observed in several regions of parkinsonian brains, including the temporal cortex, thalamus and red nucleus. However, the most pronounced increase occurred in the substantia nigra and basal nucleus. This may be due to an increase of the superoxide dismutase activity or be a result of the presence of a compound with superoxide dismutase-like activity, and may reflect the involvement of radical-induced cell damage in the pathogenesis of Parkinson's disease.