Glycine receptors (GlyRs) are members of the ligand-gated ion channel superfamily. Each subunit has four transmembrane segments (TM1-TM4). Several studies suggest that amino acids in all four TMs face into a water-filled, alcohol and anesthetic binding cavity in the extracellular portion of the transmembrane domain. TM4 should contribute a "wall" to this cavity, but the residues involved are unknown. Here, we determined the ability of an alcohol analog, propyl methanethiosulfonate (propyl MTS), to covalently react with twelve GlyR TM4 positions (I401-I412) after mutating the original amino acids to cysteines. Reactivity of a cysteine with propyl MTS implies that the cysteine is exposed to water. W407C, I409C, Y410C, and K411C showed altered receptor function following reaction with propyl MTS in the presence or absence of glycine. The cysteine mutations alone eliminated the effects of ethanol for I409C, Y410C, and K411C, and reduced the effects of octanol for I409C and isoflurane for K411C. The ability of propyl MTS to reduce isoflurane and chloroform potentiation was examined in the reactive mutants. Potentiation by isoflurane was significantly reduced for I409C after reaction. These data demonstrate water-accessibility of specific TM4 positions in the GlyR and suggest involvement of these residues with alcohol and anesthetic action.