The effects of Zn2+ on recombinant N-methyl-D-aspartate (NMDA) receptors expressed in Xenopus oocytes were examined. Zn2+ inhibited macroscopic currents induced by NMDA at both NR1/NR2B and NR1/NR2A receptors. At NR1/NR2B receptors the Zn2+ concentration-inhibition curve was monophasic, with an apparent affinity for Zn2/ of 1.6 microM, and inhibition by Zn2+ was not voltage-dependent. In contrast, the Zn2+ concentration-inhibition curve at NR1/NR2A receptors was biphasic, with high (Ki = 0.08 microM) and low (Ki = 30 microM) affinity components. The high affinity component produced a maximal inhibition of 45% of macroscopic NMDA currents and was not voltage-dependent. Thus, Zn2+ is more potent in producing voltage-independent block at NR1/NR2A than at NR1/NR2B receptors, but the maximal effect of Zn2+ is smaller at NR1/NR2A than at NR1/NR2B receptors. The low affinity component of Zn2+ inhibition at NR1/NR2A receptors was voltage-dependent and may represent an open-channel blocking effect of Zn2+. Differential effects of Zn2+ at recombinant NMDA receptors containing different NR2 subunits provide a potential marker for distinguishing subtypes of native NMDA receptors and for dissecting the site and mechanism of action of Zn2+ at these receptors.