Continuous subcutaneous administration of polyamine-modified catalase that has increased permeability at the blood-brain barrier showed both a highly significant delay in onset and an increase in survival in a transgenic mouse model of familial amyotrophic lateral sclerosis having a point mutation in the gene encoding copper/zinc superoxide dismutase. These results suggest that hydrogen peroxide-mediated oxidative stress with subsequent free radical damage involving nitric oxide and possibly hydroxyl radicals in motor neurons may be the culprit in familial amyotrophic lateral sclerosis.