Hyperphosphorylation of tau leading to aggregated tau and tangle formation is a common pathological feature of tauopathies, including Alzheimer's disease. Abnormal phosphorylation of tau by kinases, in particular GSK3 beta, has been proposed as a pathogenic mechanism in these diseases. In this study we demonstrate that the heat shock protein 90 (Hsp90) maintains the stability and function of the GSK3 beta. By using both rat primary cortical neurons and COS-7 cells, we show that Hsp90 inhibitors lead to a reduction of the protein level of GSK3 beta, and that this effect is associated with both a decrease in tau phosphorylation at putative GSK3 beta sites and an induction in heat shock protein 70 (Hsp70) levels. We further show that Hsp90 associates with the GSK3 beta regulating its stability and function and preventing its degradation by the proteasome.