Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by selective motor neuronal death. The cause of ALS is unclear, but accumulating evidence, such as the insufficient clearance of glutamate through the glutamate transporter, and the specific distribution of Ca2+-permeable AMPA receptors in spinal motor neurons, indicates that glutamate-induced neurotoxicity is involved in its pathogenesis. Interestingly, nitric oxide (NO), which has been identified as an endothelium-derived relaxing factor (EDRF), was found to be a pivotal inducer of glutamate-induced neuronal death. NO is generated by nitric oxide synthase (NOS), of which there are three subtypes: neuronal NOS expressed mainly in neurons, inducible NOS in astroglia, and endothelial NOS in vessels. NO-related toxicity is caused by peroxynitrite, formed by the reaction of NO with superoxide anions, resulting in the nitration of tyrosine residues in neurofilaments, irreversible inhibition of the mitochondrial respiratory chain, and inhibition of the glutamate transporter. Clinically, the axonal spheroids of motor neurons are reported to be immunoreactive to anti-nitrotyrosine antibody, and there are elevated levels of the metabolites of NO in the cerebrospinal fluid of ALS patients. Since physiologically normal motor neurons express limited amounts of neuronal NOS, the source of NO is considered to be non-motor neurons expressing neuronal NOS, astroglia expressing inducible NOS, or motor neurons themselves inducing neuronal NOS. Conversely, neurons containing neuronal NOS are known to be resistant to toxic stimuli, which raises the possibility that such neurons are protected by NO. Several mechanisms have been reported to mediate the NO-related neuroprotection, including cyclic guanosine 3',5'-monophosphate (cyclic GMP), a downstream product of NO generation. This review summarizes previous studies on NO, focusing on its dual functions of neurotoxicity or neuroprotection, and discusses the putative roles of NO in relation to the pathogenesis of ALS.