To test the novel hypothesis that the K⁺ efflux mediated by NMDA receptors might be regulated differently than the influx of Ca²⁺ and Na⁺ through the same receptor channels, NMDA receptor whole-cell currents carried concurrently or individually by Ca²⁺, Na⁺ and K⁺ were analysed in cultured mouse cortical neurons. In contrast to the NMDA inward current carried by Ca²⁺ and Na⁺, the NMDA receptor outward K⁺ current or NMDA-K current, recorded either in the presence or absence of extracellular Ca²⁺ and Na⁺, and at different or the same membrane potentials, showed much less sensitivity to alterations in intracellular Ca²⁺ concentration and underwent little rundown. In line with a selective regulation of the NMDA receptor K⁺ permeability, the ratio of the NMDA inward/outward currents decreased, and the reversal potential of composite NMDA currents recorded in physiological solutions shifted by -8.5 mV after repeated activation of NMDA receptors. Moreover, a depolarizing pre-pulse of a few seconds or a brief burst of depolarizing pulses selectively augmented the subsequent NMDA-K current, but not the NMDA inward current. On the other hand, a hyperpolarizing pre-pulse showed the opposite effect of reducing the NMDA-K current. The voltage-activity-dependent regulation of the NMDA-K current did not require the existence of extracellular Ca²⁺ or Ca²⁺ influx; it was, however, affected by the duration of the pre-pulse and was subject to a time-dependent decay. The burst of excitatory activity revealed a lasting upregulation of the NMDA-K current even 5 s after termination of the pre-pulses. Our data reveal a selective regulation of the NMDA receptor K⁺ permeability and represent a novel model of voltage-activity-dependent plasticity at the receptor level.