In oocytes, glycine activates receptors formed by diheteromeric combinations of N-methyl-d-aspartate (NMDA) NR1 and NR3 subunits. In contrast, functional receptors in mammalian cells require the simultaneous expression of NR1 and both NR3A and NR3B subunits. In vivo, NR3A and NR3B subunits show differential expression patterns and thus may not naturally form triheteromeric receptors. In this study, we examined whether NR1 splice variants play a role in allowing assembly of functional diheteromeric receptors in mammalian cells. Little current was found in human embryonic kidney 293 cells coexpressing either NR3A or NR3B and the NR1-1a splice variant. However, robust glycine-activated currents were generated in cells transfected with NR3(A or B) and either NR1-2a, NR1-3a, or NR1-4a, and current density was correlated with NR1 C-terminal length. Truncation of the NR1-1a C terminus modestly enhanced NR1-1a/NR3A currents, whereas only small increases were observed with mutations of C-terminal residues that control trafficking or phosphorylation. In contrast, large currents were observed when an extracellular phenylalanine in NR1-1a that influences glycine access was mutated to alanine. A separate mutation in NR1-1a that disrupts glycine binding did not generate responses in NR1-1a/NR3A receptors alone, but it produced a greater than 30-fold potentiation of currents during coapplication of glycine and the glycine antagonist 7-chlorokynurenic acid. Finally, transfection of cells with the NR1-4a subunit along with NR2 and NR3 subunits resulted in the expression of both NR1/NR3 receptors and conventional NMDA receptor currents. These results indicate a prominent role for NR1 splice variants in the functional expression of NR1/NR3 receptors in mammalian cells.