The discovery that some cases of familial amyotrophic lateral sclerosis (FALS) are associated with mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1) has focused much attention on the function of SOD1 as related to motor neuron survival. Here we describe the creation and characterization of mice completely deficient for this enzyme. These animals develop normally and show no overt motor deficits by 6 months in age. Histological examination of the spinal cord reveals no signs of pathology in animals 4 months in age. However Cu/Zn SOD–deficient mice exhibit marked vulnerability to motor neuron loss after axonal injury. These results indicate that Cu/Zn SOD is not necessary for normal motor neuron development and function but is required under physiologically stressful conditions following injury.