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This article has not been copyedited and formatted. The final version may differ from this version. Abstract NMDA receptors are glutamate-and glycine-gated channels that mediate fast excitatory transmission in the central nervous system and are critical to synaptic development, plasticity and integration. They have a rich complement of modulatory sites, which represent important pharmacologic targets. Ifenprodil is a well-tolerated NMDA receptor inhibitor; it is selective for GluN2B-containing receptors; and has neuroprotective effects. The mechanism by which ifenprodil inhibits NMDA receptor responses is not fully understood. The inhibition is incomplete and non-competitive with other known NMDA receptor agonists or modulators, although reciprocal effects have been reported between ifenprodil potency and that of extracellular ligands including glutamate, glycine, zinc, protons and polyamines. Recently, structural studies revealed that ifenprodil binds to a unique site at the interface between the extracellular N-termini of GluN1 and GluN2B subunits supporting the view that interactions with other extracellular modulators are indirect. In this study, we examined how ifenprodil affects the gating reaction of NMDA receptors in conditions designed to minimize actions by contemporaneous ligands. We found that ifenprodil decreased NMDA receptor equilibrium open probability by raising an energetic barrier to activation and also by biasing the receptor toward low open probability gating modes. These results demonstrate intrinsic effects of ifenprodil on NMDA receptor stationary gating kinetics and provide means to anticipate how ifenprodil will affect receptor responses in defined physiologic and pathologic circumstances.