There have been no reports of changes in free radical inactivating enzymes in the anterior horn of the spinal cord in ALS despite great interest in the possibility that free radicals might be important in the aetiology of the disease. In this study we have measured copper/zinc superoxide dismutase (Cu/ZnSOD), manganese superoxide dismutase (MnSOD) and glutathione peroxidase (GSHPX) activities in anterior horn tissue obtained from patients with ALS and from controls. Total SOD activity was no different in the anterior horn of ALS cases compared to controls, but Cu/ZnSOD activity was reduced, and that of MnSOD increased, at thoracic cord level only. No detectable activity of GSHPX or cytochrome P450 (unpublished data) was found. These latter negative findings are important because they suggest that generation of free radicals from exogenous chemicals is not important in ALS and further that the neurone (as compared to other cell types) is poorly protected against the toxicity of hydrogen peroxide.