The M2 loop and the M3 segment are the major pore-lining domains in the GluR channel. These domains determine ion permeation and channel block processes and are extensively involved in gating. To study the distribution of the membrane electric potential across the GluR channel pore, we recorded from alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptors containing M2 and M3 cysteine substitutions in the GluR-A subunit and measured the voltage dependence of the modification rate of these substituted cysteines by methanethiosulfonate reagents either in the presence or absence of glutamate. In the presence of glutamate, the voltage dependence became gradually stronger for positions located deeper in the pore suggesting that the electrostatic potential drops fairly uniformly across the pore in the open state. In contrast, in the absence of glutamate, the voltage dependence was biphasic. The difference in the electrostatic potential in the presence and absence of glutamate had an apparent maximum in the middle of the extracellular vestibule. We suggest that these state-dependent changes in the membrane electric potential reflect a reorientation of the dipoles of the M2 loop alpha-helices toward and away from the center of the channel pore during gating.