Humans exposed to excess levels of manganese (Mn 2+ ) express psychiatric problems and deficits in attention and learning and memory. However, there is a paucity of knowledge on molecular mechanisms by which Mn 2+ produces such effects. We now report that Mn 2+ is a potent inhibitor of [ 3 H]-MK-801 binding to the NMDA receptor channel in rat neuronal membrane preparations. The inhibition of [ 3 H]-MK-801 to the NMDA receptor channel by Mn 2+ was activity-dependent since Mn 2+ was a more potent inhibitor in the presence of the NMDA receptor co-agonists glutamate and glycine (K i = 35.9 ± 3.1 μM) than in their absence (K i = 157.1 ± 6.5 μM). We also show that Mn 2+ is a NMDA receptor channel blocker since its inhibition of [ 3 H]-MK-801 binding to the NMDA receptor channel is competitive in nature. That is, Mn 2+ significantly increased the affinity constant (K d ) with no significant effect on the maximal number of [ 3 H]-MK-801 binding sites (B max ). Under stimulating conditions, Mn 2+ was equipotent in inhibiting [ 3 H]-MK-801 binding to NMDA receptors expressed in neuronal membrane preparations from different brain regions. However, under basal, non-stimulated conditions, Mn 2+ was more potent in inhibiting NMDA receptors in the cerebellum than other brain regions. We have previously shown that chronic Mn 2+ exposure in non-human primates increases Cu 2+ , but not zinc or iron concentrations in the basal ganglia (Guilarte et al., Experimental Neurology 202: 381-390, 2006). Therefore, we also tested the inhibitory effects of Cu 2+ on [ 3 H]-MK-801 binding to the NMDA receptor channel. The data shows that Cu 2+ in the presence of glutamate and glycine is a more potent inhibitor of the NMDA receptor than Mn 2+ . Our findings suggest that the inhibitory effect of Mn 2+ and/or Cu 2+ on the NMDA receptor may produce a deficit in glutamatergic transmission in the brain of individuals exposed to excess levels of Mn 2+ and produce neurological dysfunction.