In the present study, we analysed changes in the expression, subcellular distribution and phosphorylation state of the microtubule-associated protein tau and other cytoskeletal proteins after neurone-specific activation of the mitogen-activated protein kinase (MAPK) in the CNS in vivo. We used transgenic mice with a neurone-specific expression of activated ras protein (p21H-ras(Val12), synapsin I promoter) that is associated with an augmented activity of the MAPK. Chronic activation of MAPK cascade influenced tau protein phosphorylation, localisation and dendritic morphology. While the amount of tau protein was elevated by 9%, phospho-epitopes detected by the monoclonal antibodies AT270, 12E8 and SMI34 were increased by about 21%, 40% and 59% respectively. Steady-state levels of tau mRNA were not affected. Thus, the increase in tau protein was most likely due to stabilisation of tau protein by augmented phosphorylation. While in wild-type animals tau protein was preferentially localised in axons, a prominent immunoreactivity was found in the somatodendritic compartment of transgenic mice. This subcellular translocation typically seen in pyramidal neurones was associated with an increase in the dendritic calibre by about 30% and is paralleled by an increase in tubulin of 19%. We were unable to obtain any morphological indication of neurodegenerative processes in these animals. We suggest that the moderate increase in tau protein and phosphorylation may be part of the neuroprotective mechanism. However, further studies on aged transgenic mice will be necessary to establish potential effects on neuronal viability.