When deprived of trophic factors, the majority of cultured motor neurons undergo nitric oxide-dependent apoptosis. However, for reasons that have remained unclear, 30-50% of the motor neurons survive for several days without trophic factors. Here we hypothesize that the resistance of this motor neuron subpopulation to trophic factor deprivation can be attributed to diminished nitric oxide production resulting from the activity of the arginine-degrading enzyme arginase. When incubated with nor-N(G)-hydroxy-nor-L-arginine (NOHA), the normally resistant trophic factor-deprived motor neurons showed a drop in survival rates, whereas trophic factor-treated neurons did not. NOHA-induced motor neuron death was inhibited by blocking nitric oxide synthesis and the scavenging of superoxide and peroxynitrite, suggesting that peroxynitrite mediates NOHA toxicity. When we transfected arginase 1 into motor neurons to see whether it alone could abrogate trophic factor deprivation-induced death, we found that its forced expression did indeed do so. The protection afforded by arginase 1 expression is reversed when cells are incubated with NOHA or with low concentrations of nitric oxide. These results reveal that arginase acts as a central regulator of trophic factor-deprived motor neuron survival by suppressing nitric oxide production and the consequent peroxynitrite toxicity. They also suggest that the resistance of motor neuron subpopulations to trophic factor deprivation may result from increased arginase activity.