Oxidative stress has been implicated in the pathogenesis of several neurological disorders. We examined the regional distribution of copper/zinc superoxide dismutase (SOD-1), one of the key antioxidant enzymes, in the human central nervous system using in situ hybridization. Our results show that the enzyme is present at high levels of constitutive expression in alpha-motor neurons, oculomotor neurons, nucleus basalis, substantia nigra, neocortex, and the hippocampal sector resistant to hypoxia (H2). Relatively lower levels were found in Sommer's sector (H1) and Purkinje cells. We conclude that a lower constitutive level of SOD-1 expression may play a role in the selective vulnerability of certain neuronal populations to hypoxia but does not correlate with the patterns of neurodegeneration observed in amyotrophic lateral sclerosis. Parkinson's disease, and Alzheimer's disease.