THE structure of glutamate receptor–channel (GluR) subunits has recently been shown to differ from that of other ligand-gated channels and to contain a voltage-gated channel-like pore-forming motif. The view that the structure of GluR complexes is similar to the pentameric structure of other ligand-gated channels was questioned here. Studies of the response properties of the GluR1 subunit of the AMPA subtype of GluRs, co-expressinXenopusoocytes with its L646A mutant, which differs only by a greatly reduced sensitivity to quisqualate, provide new evidence suggesting that the GluR1 homomeric receptor channel has a tetrameric structure.