We have been examining the interaction of a local anesthetic derivative, QX-222, with the ion channel pore of the muscle AChR, using a combination of mutagenesis, oocyte expression, and electrophysiology. Single channel recording, together with macroscopic voltage-jump relaxations, provides a measure of the residence time of the open channel blocker within the pore. We have found systematic changes in the apparent affinity of the open channel for QX-222 following amino acid substitutions in the proposed M2 transmembrane helix of each of the four subunits of the AChR. Assigning the number 1' to the residue at the cytoplasmic end of the M2 helix, positions 2',6',10',14', and 18' are modeled as forming the lining of the pore. Polar to nonpolar substitutions at 6' decrease QX-222 residence time, while the opposite effect is seen at position 10'. Nonpolar to polar substitutions have the converse effect. The distance between the aromatic and quaternary amine moieties of QX-222 corresponds almost exactly to the repeat distance of an alpha helix. This structural feature is common to many local anesthetic drugs. We propose a model for the binding of QX-222 within the ion channel of the AChR that is consistent with these observations.