Clinical concentrations of pentobarbital inhibit the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-selective glutamate receptor (GluR) channels. Recently, the AMPA-selective GluR channels that contained the α2 subunit were shown to be more sensitive to pentobarbital block than those without the α2 subunit. Here we demonstrated that replacement by glutamine of the arginine residue in putative transmembrane segment M2 of the α2 subunit (mutation α2-R586Q) drastically reduced the pentobarbital sensitivity of the α2 heteromeric channel to the level comparable to those of the α1 and α2-R586Q homomeric channels. These results suggest that the arginine residue in segment M2 of the α2 subunit is the critical determinant of the sensitivities of the AMPA-selective GluR channels to pentobarbital. © 1995.