The microtubule-associated protein tau is present in the pathologic hallmarks of Alzheimer's disease and its production and deposition have been implicated in the pathogenesis of the disease. We detected tau mRNA using in situ hybridization histochemistry in the hippocampus, visual cortex, and cerebellum, and compared its level in Alzheimer's disease with controls. The amount of tau mRNA also was determined as a ratio of total polyadenylated mRNA in each area. A significant and gene-specific increase in tau mRNA hybridization was found in hippocampal fields CA4 and CA3, with a similar trend in the dentate gyrus. In contrast, no change was found in the visual cortex or cerebellum in Alzheimer's disease. Increased hippocampal expression of tau mRNA also was present in cases of non-Alzheimer's dementia. Enhanced tau mRNA may be a marker of attempted plasticity involving the cytoskeleton in neuronal populations affected by various neurodegenerative disorders.