Previous studies had not excluded the possibility that the mechanism by which Xenon (Xe) blocks N-methyl-D-aspartate (NMDA) receptors might be that of an open-channel blocker. We tested this possibility on mutant NMDA receptors carrying an alanine (A) to cysteine (C) mutation located within the SYTANLAAF-motif of the third transmembrane region (TM3). This mutation was shown to yield constitutively open ion channels after modification with a thiol-modifying reagent. We expressed such mutant channels in Neuro2A cells and recorded glutamate (50 microM)-induced currents in the whole cell recording mode. Although Xe (3.5 mM) blocked the currents through the wild-type receptor NR1-1a/NR2A and NR1-1a/NR2B by approximately 40% and those through the mutant receptors NR1-1a/NR2A(A650C) or NR1-1a/NR2B(A651C) by approximately 30%, it was unable to block the currents through the methane thiosulfonate etyhlammonium-modified mutant receptors. On the other hand, established open-channel blockers of the NMDA receptor such as MK-801 (1 microM) or Mg ions (Mg(2+); 1 mM) were able to block these permanently open channels. These results suggest that Xe does not act as a classical open-channel blocker at the NMDA receptor.