One of the hallmarks of Alzheimer's Disease is the presence of abundant neurofibrillary tangles (NFTs) in the brains of affected individuals. Hyperphosphorylated tau is a major component of paired helical filaments (PHFs) in NFTs. Tau is a neuronal microtubule associated protein found primarily in axons. Normal tau promotes tubulin polymerization and stabilizes microtubule (MT) structures, whereas hyperphosphorylated tau reduces its affinity for MTs and destabilizes MT-structures. This results in the disruption of vital cellular processes (e.g. axonal transport) and leads to the degeneration of affected neurons. Processes leading to the hyperphosphorylation of tau and formation of neurofibrillary lesions in Alzheimer's Disease (AD) brains are not understood. Phosphorylation of a substrate molecule like tau depends upon the equilibrium between kinase and phosphatase activities and the availability of their substrate molecules in a given system. Therefore, to understand the relative roles of kinase and phosphatase activities, we studied the long-term kinetics of phosphorylation in AD and control brain extracts in the presence and absence of the phosphatase inhibitor okadaic acid (OA) using histone, casein and bacterially expressed tau as exogenous substrates. It was found that both kinase and phosphatase activities were higher in AD compared to control brains. Surprisingly, between 18 and 24 hours, there was a robust increase in phosphorylation of endogenous proteins in the brain extracts only when bacterially expressed tau was present in the phosphorylation reaction mixture. This pattern of phosphorylation activity was unaffected by OA. Significant difference in the phosphorylation of tau isoforms was also seen during this period. These data suggest that the expression and differential phosphorylation of certain tau isoforms may be responsible for the robust increase in phosphorylation and may play an important role in Alzheimer's pathology.