N-Methyl-d-aspartate receptors (NMDARs) are ligand-gated ion channels that play an important role in neuronal development, plasticity, and excitotoxicity. NMDAR antagonists are neuroprotective in animal models of neuronal diseases, and the NMDAR open-channel blocker memantine is used to treat Alzheimer's disease. In view of the clinical application of these pharmaceuticals and the reported expression of NMDARs in immune cells, we analyzed the drug's effects on T-cell function. NMDAR antagonists inhibited antigen-specific T-cell proliferation and cytotoxicity of T cells and the migration of the cells toward chemokines. These activities correlated with a reduction in T-cell receptor (TCR)-induced Ca2+ mobilization and nuclear localization of NFATc1, and they attenuated the activation of Erk1/2 and Akt. In the presence of antagonists, Th1 effector cells produced less interleukin-2 (IL-2) and gamma interferon (IFN-γ), whereas Th2 cells produced more IL-10 and IL-13. However, in NMDAR knockout mice, the presumptive expression of functional NMDARs in wild-type T cells was inconclusive. Instead, inhibition of NMDAR antagonists on the conductivity of Kv1.3 and KCa3.1 potassium channels was found. Hence, NMDAR antagonists are potent immunosuppressants with therapeutic potential in the treatment of immune diseases, but their effects on T cells have to be considered in that Kv1.3 and KCa3.1 channels are their major effectors.