The cellular localization of copper-zinc superoxide dismutase (CuZn SOD) mRNA was determined in the human hippocampus by in situ hybridization with a 35S-labelled DNA probe complementary to human CuZn SOD mRNA. A positive hybridization signal was detected in pyramidal cell layers CA1-CA4 of Ammon's horn (CA), pyramidal cells of subiculum and in the granule cells of the dentate gyrus. The fact that CuZn SOD gene expression is important in neurones which are preferentially vulnerable in neurodegenerative processes such as Alzheimer's disease, suggests a role played by oxygen free radicals in the mechanism of nerve cell death.