d-Tubocurarine is a potent competitive antagonist of both the muscle-type nicotinic acetylcholine receptor (AChR) and the serotonin type-3 receptor (5HT(3)R). We have previously used a series of structural analogs of d-tubocurarine to demonstrate that the ligand-binding domains of both receptors share common structural features. We have now extended these studies to examine the interaction of a series of d-tubocurarine analogs with 5HT(3)Rs containing mutations at either of two residues within the ligand-binding domain of the receptor (W90F and R92A). The W90F mutation results in an approximately 2-4-fold decrease in the affinity of the analogs relative to wild-type receptors, while the R92A results in an approximately 8-10-fold increase in affinity. However, since the effect of a given mutation is more or less equivalent for all analogs, neither residue W90 nor R92 is likely to make a specific interaction with d-tubocurarine itself. Rather, these two residues are likely to play a role in determining both the geometry of the binding site, as well as the overall environment that a ligand encounters in the binding site.