Abnormal phosphorylation of tau is a feature of Alzheimer's disease (AD), which develops prematurely in Down syndrome (DS) patients. Cognitive impairment is also recognized as a clinical characteristic of schizophrenia, which does not appear to be associated with tau-aggregate formation. Several kinases can phosphorylate tau in cell-free assays. Here we show increased activity of mitogen-activated protein kinases (MAPKs) (including ERK1/2, SAPKs and p38) in post mortem AD and DS brains, which could not be accounted for by expression changes. In contrast, glycogen synthase kinase-3 activity (GSK-3 alpha beta) was reduced significantly. Examination of tau in AD and DS using antibodies selective for MAPK phosphorylation sites showed increased immunoreactivity. In addition, phosphorylation of S(199), reportedly a selective substrate for cyclin-dependent kinase-5 (cdk5) or GSK-3 alpha beta was only observed in AD samples, which showed a concomitant increase in the expression of p25, the enhancing cofactor for cdk5 activity. However, in schizophrenia brain, MAPK-phosphorylated tau was unchanged compared to matched controls, despite similar expression levels to those in AD. The activities of the MAPKs and GSK-3 alpha beta were also unchanged. These data demonstrate that in AD and DS, enhanced MAPK activity, which has an established role in regulating neuronal plasticity and survival, can account for irregular tau phosphorylation, and that the molecular processes involved in these neurodegenerative disorders are distinct from those in schizophrenia. These data also question the significance of GSK-3 alpha beta, as much previous work carried out in vitro has placed this kinase as a favoured candidate for involvement in the pathological phosphorylation of tau.