The discovery of missense mutations leading to reduced enzymatic activity in the copper/zinc superoxide dismutase (SOD1) in human familial amyotrophic lateral sclerosis has heightened interest in the role of free radicals in neurodegenerations but left the mechanisms by which they may cause neuronal death unexplained. We have approached this problem by specifically inhibiting the synthesis of SOD1 in cultured PC12 cells with antisense oligonucleotides. Cell survival in both untreated and nerve growth factor (NGF)-treated PC12 cells was inhibited by down-regulation of SOD1, with NGF-treated cells dying at lower levels of inhibition than untreated cells. Dying cells showed DNA degradation characteristic of apoptosis and could be rescued by the antioxidant vitamin E, with a combination of vitamin E and NGF being most efficacious. These results suggest that the induction of cell death by inhibition of SOD1 is due to free radical induction of apoptosis and that growth factor therapy for free-radical-mediated disease may require antioxidants in order to be effective.