The objective of this study was to asses the response of the microtubule-associated protein tau to acute rise in the concentration of free cytoplasmic calcium ([Ca2+]i) in rat cortical neurons and mouse cerebellar granule cells in culture. One-hour exposure to glutamate (100 microM), N-methyl-D-aspartate (100 microM), KCl (50 mM), and ionomycin (5 microM) led to tau protein dephosphorylation as indicated by an appearance of additional faster moving bands on Western immunoblots with a phosphorylation-independent antibody and an increase in the tau-1 immunoreactivity associated with the appearance of an additional faster moving band. Lowering the extracellular concentration of Ca2+ to less than 1 microM fully prevented the drug-induced tau protein dephosphorylation indicating a dependence on Ca2+ influx from the extracellular environment. Administration of okadaic acid (inhibitor of phosphatase 1/2A) simultaneously with the above mentioned drugs decreased the drug-mediated dephosphorylation. Pre-incubation with okadaic acid fully prevented the dephosphorylation. Treatment with cypermethrin (inhibitor of phosphatase 2B) was without effect when administered either alone, simultaneously with the drugs, or pre-incubated. These findings indicate that, independently of the influx pathway, [Ca2+]i elevation leads to dephosphorylation of the microtubule-associated protein tau and implicate phosphatase 1 and/or 2A in the process.