Accumulation of paired helical filaments (PHFs) in neurofibrillary tangles, neuropil threads, and dystrophic neurites is one of the major neuropathological hallmarks of Alzheimer disease (AD). The principal protein subunit of PHFs is the abnormally hyperphosphorylated tau. Glycogen synthase kinase 3beta (GSK-3beta) is one of the candidate kinases involved in PHF-tau formation. To play a role in PHF-tau formation, it would be expected that GSK-3beta is active in tangle bearing neurons. In the present study, we investigated the regional and intracellular distributions of active and inactive forms of GSK-3beta in brains staged for neurofibrillary changes. We found that neurons with tangle-like inclusions positive for active, but not inactive, GSK-3beta appear initially in the Pre-alpha layer of the entorhinal cortex and extend to other brain regions, coincident with the sequence of the development of neurofibrillary changes. Active, but not inactive, GSK-3beta was found to initially accumulate in the cytoplasm of pretangle neurons. These data provide direct in situ evidence that is consistent with the involvement of GSK-3beta in PHF-tau formation.