Delayed excitotoxic neuronal death following insult from exposure to high glutamate concentrations appears important in several CNS disorders. Although delayed excitotoxicity is known to depend on NMDA receptor (NMDAR) activity and Ca²⁺ elevation, the electrophysiological mechanisms underlying post-insult persistence of NMDAR activation are not well understood. Membrane depolarization and non-specific cationic current in the post-insult period have been reported previously, but were not sensitive to NMDAR antagonists. Here, we analyzed mechanisms of the post-insult period using parallel current- and voltage-clamp recording and Ca²⁺ imaging in primary hippocampal cultured neurons. We also compared more vulnerable older neurons (~22 days in vitro, DIV) to more resistant younger (~15 DIV) neurons, to identify processes selectively associated with cell death in older neurons. During exposure to a modest glutamate insult (20 µM, 5 min), similar degrees of Ca²⁺ elevation, membrane depolarization, action potential block, and increased inward current occurred in younger and older neurons. However, following glutamate withdrawal, these processes recovered rapidly in younger but not in older neurons. The latter also exhibited a concurrent post-insult increase in spontaneous miniature excitatory post-synaptic currents, reflecting glutamate release. Importantly, post-insult NMDAR antagonist administration reversed all of these persisting responses in older cells. Conversely, repolarization of the membrane by voltage clamp immediately after glutamate exposure reversed the NMDAR-dependent Ca²⁺ elevation. Together, these data suggest that, in vulnerable neurons, excitotoxic insult induces a sustained positive feedback loop between NMDAR-dependent current and depolarization-mediated glutamate release which persists after withdrawal of exogenous glutamate and drives Ca²⁺ elevation and delayed excitotoxicity.