NMDA receptor activity is modulated by various compounds, including sulfhydryl redox agents and Zn(2+). In addition to a slow and persistent component of redox modulation common to all NMDA receptors, NR1/NR2A receptors uniquely have a rapid and reversible component that has been variously attributed to redox or Zn(2+) effects. Here we show that this rapid modulatory effect can be described by two time constants with relatively fast ( approximately 6 sec) and intermediate (60 sec) half lives, and it is likely to be attributable to both redox agents and Zn(2+). Using site-directed mutagenesis, we identified three pairs of cysteine residues that underlie the various kinetic components of redox modulation of NMDA-evoked currents in Xenopus oocytes expressing NR1/NR2A receptors: (1) Cys 87 and Cys 320 in NR2A underlie the fast component, (2) Cys 79 and Cys 308 in NR1 underlie the intermediate component, and (3) Cys 744 and Cys 798 in NR1 underlie the persistent component. Mutation of these redox-sensitive cysteine residues also affects high-affinity, voltage-independent Zn(2+) inhibition that is specific to NR1/NR2A receptors. Exposure to methanethiosulfonate agents that modify cysteine residues did not block the Zn(2+) inhibition. Thus, these cysteine residues do not appear to coordinate Zn(2+) directly. Instead, the redox status of these cysteine residues may modulate the sensitivity of the receptor to Zn(2+).