The neuropathological features associated with Alzheimer's disease (AD) brain include the presence of extracellular neuritic plaques composed of amyloid β protein (Aβ), intracellular neurofibrillary tangles containing phosphorylated tau protein and the loss of basal forebrain cholinergic neurons which innervate regions such as the hippocampus and the cortex. Studies of the pathological changes that characterize AD and several other lines of evidence indicate that Aβ accumulation in vivo may initiate phosphorylation of tau protein, which by disrupting neuronal network may trigger the process of neurodegeneration observed in AD brains. However, the underlying cause of degeneration of the basal forebrain cholinergic neurons and their association, if any, to Aβ peptides or phosphorylated tau remains mostly unknown. In the present study, using rat primary septal cultures, we have shown that aggregated Aβ peptides, in a time (18-96 h)- and concentration (0.7-60 μM)-dependent manner, induce toxicity and decrease choline acetyltransferase enzyme activity in cultured neurons. Using immunocytochemistry and immunoblotting, we have also demonstrated that Aβ treatment can significantly increase the phosphorylation of tau protein in septal cultures. At the cellular level, hyperphosphorylated tau is mostly apparent in the somatodendritic compartment of the neurons. Aβ peptide (10 μM), in addition to tau phosphorylation, also activates mitogen-activated protein kinase and glycogen synthase kinase-3β, the two kinases which are known to be involved in the formation of hyperphosphorylated tau in the AD brain. Exposure to specific inhibitors of the mitogen-activated protein kinase (i.e. PD98059) or glycogen synthase kinase-3β (i.e. LiCl) attenuated the hyperphosphorylation of the tau protein in cultured neurons. Given the evidence that tau phosphorylation can induce cell loss by disrupting neuronal cytoskeleton, it is likely that aggregated Aβ peptide triggers degeneration of septal neurons, including those expressing the cholinergic phenotype, by phosphorylation of the tau protein activated by mitogen-activated protein kinase and glycogen synthase kinase-3β. These results, taken together, suggest that cultured septal cholinergic neurons are vulnerable to Aβ-mediated toxicity and tau phosphorylation may play an important role in Aβ-induced neurodegeneration. © 2002 IBRO. Published by Elsevier Science Ltd. All rights reserved.