Alzheimer's disease presents morphologically with senile plaques, primarily made of extracellular amyloid-beta (A beta) deposits, and neurofibrillary lesions, which consist of intracellular aggregates of hyper-phosphorylated tau protein. To study the in vivo induction of tau pathology, dilute brain extracts from aged A beta-depositing APP23 transgenic mice were intracerebrally infused in young B6/P301L tau transgenic mice. Six months after the infusion, tau pathology was induced in the injected hippocampus but also in brain regions well beyond the injection sites such as the entorhinal cortex and amygdala, areas with neuronal projection to the injection site. No or only modest tau induction was observed when brain extracts from aged nontransgenic control mice and aged tau-depositing B6/P301L transgenic mice were infused. To further study A beta-induced tau lesions B6/P301L tau transgenic mice were crossed with APP23 mice. Although A beta. deposition in double-transgenic mice did not differ from single APP23 transgenic mice, double-transgenic mice revealed increased tau pathology compared to single B6/P301L tau transgenic mice predominately in areas with high A beta plaque load. The present results suggest that both extract-derived A beta species and deposited fibrillary A beta can induce the formation of tau neurofibrillary pathology. The observation that infused A beta can trigger the tau pathology in the absence of A beta deposits provides an explanation for the discrepancy between the neuroanatomical location of A beta deposits and the development and spreading of tau lesions in Alzheimer's disease brain.