5-HT₃ and GABA_C receptors are members of the Cys-loop superfamily of neurotransmitter receptors, which also includes nicotinic acetylcholine (nACh), GABA_A and glycine receptors. The details of how agonist binding to these receptors results in channel opening is not fully understood, but is known to involve charged residues at the extracellular/transmembrane interface. Here we have examined the roles of such residues in 5-HT₃ and GABA_C receptors. Charge reversal experiments, combined with data from activation by the partial agonist -alanine, show that in GABA_C receptors there is a salt bridge between Glu92 (in loop 2) and Arg258 (in the pre-M1 region), which is invovled in receptor gating. The equivalent residues in the 5-HT₃ are important for receptor expression, but charge reversal experiments do not restore function indicating there is not a salt bridge here. There is, however, an interaction between Glu215 (loop 9) and Arg246 (Pre M1) in the 5-HT₃ receptor, although the coupling energy determined from mutant cycle analysis is lower than might be expected for a salt bridge. Overall the data show that charged residues at the extracellular/transmembrane domain interfaces in 5-HT₃ and GABA_C receptors are important, and that specific, but not equivalent, molecular interactions between them are involved in the gating process. Thus we propose that the molecular details of interactions in the transduction pathway between the binding site and the pore can differ between different Cys-loop receptors.