Modulation of NMDA-mediated responses by oxidizing and reducing reagents has been described in a variety of neuronal preparations. Here, we report that NMDA-gated currents of oocytes expressing heteromeric NMDA receptors are also modulated by sulfhydryl redox reagents. Each cysteine residue in the NMDAR1 (NR1) subunit and each conserved NMDAR2 (NR2) cysteine residue in a prototypical subunit (NR2B) was tested for its role in redox modulation. We have identified 2 cysteines in the NR1 subunit that are required for redox modulation of NMDA-gated currents in oocytes expressing NR1-NR2B, NR1-NR2C, or NR1-NR2D receptors. Mutation of these same 2 cysteines also eliminated potentiation by spermine and shifted the IC50 for H+ inhibition and the EC50 for NMDA. Redox modulation of heteromeric NR1-NR2A receptors appeared to be different from that of the other heteromeric receptors, indicating the presence of one or more unique redox modulatory sites on NR1-NR2A receptors.