Sublethal ischemic challenges can protect neurons against a second, more severe hypoxic insult. We report here that nonlethal chemical ischemia induces a transient alteration of NMDA receptors in rat cortical neurons in culture. Cells were incubated with 3 mM KCN in a glucose-free solution for 90 min. Analysis of NMDA receptor unitary events in patches excised from KCN-treated neurons showed an increased incidence of a small conductance channel 24 h after chemical ischemia. Whole-cell recordings of NMDA-induced currents 1 day after cyanide exposure revealed a significant increase in voltage-dependent extracellular Mg(2+) block compared with untreated neurons. The block reverted to control levels within 48 h. Both of these changes in the NMDA receptor could decrease the overall current flowing through the channel. Message levels for the NMDA receptor subunits NR1, NR2A, and NR2B were not different between the chemically challenged neurons and control cells, whereas NR2C message was barely detectable in either group. These results suggest that the alterations in NMDA receptor properties after KCN exposure may contribute to the molecular mechanisms that are activated in neurons to withstand lethal ischemic events in the brain after preconditioning.