Single channel currents from recombinant N-methyl-D-aspartate (NMDA) receptors having an N-to-Q mutation in M2 reveal a divalent cation binding site that is near the entrance of the pore (∼0.2 through the electric field). Ca²⁺ rapidly binds to this site and readily permeates the channel, while Mg²⁺ binds more slowly and does not permeate as readily. In wild-type receptors, Mg²⁺ also blocks the current by occupying a site that is ∼0.6 through the field. When the more external site is occupied by Ca²⁺, the conductance of the pore to Na⁺ is reduced but not abolished, perhaps by an electrostatic blocking mechanism. The site serves to enrich the fraction of NMDA receptor current carried by Ca²⁺.