The altered function and/or structure of tau protein is postulated to cause cell death in tauopathies and Alzheimer's disease. However, the mechanisms by which tau induces neuronal death remain unclear. Here we show that overexpression of human tau and of some of its N-terminal fragments in primary neuronal cultures leads to an N-methyl-D-aspartate receptor (NMDAR)-mediated and caspase-independent cell death. Death signaling likely originates from stimulation of extrasynaptic NR2B-subunit-containing NMDARs because it is accompanied by dephosphorylation of cAMP-response-element-binding protein (CREB) and it is inhibited by ifenprodil. Interestingly, activation of NMDAR leads to a crucial, sustained, and delayed phosphorylation of extracellular-regulated kinases 1 and 2, whose inhibition largely prevents tau-induced neuronal death. Moreover, NMDAR involvement causes the fatal activation of calpain, which, in turn, degrades tau protein into a 17-kDa peptide and possibly other highly toxic N-terminal peptides. Some of these peptides are hypothesized, on the basis of our in vitro experiments, to initiate a negative loop, ultimately leading to cell death. Thus, inhibition of calpain largely prevents tau degradation and cell death. Our findings unravel a cellular mechanism linking tau toxicity to NMDAR activation and might be relevant to Alzheimer's disease and tauopathies where NMDAR-mediated toxicity is postulated to play a pivotal role.