The instant invention features compounds, for example, 5-HT.sub.3 receptor antagonists, having a peripherally restricted mode of action such that the compounds affect 5-HT.sub.3 receptors of the peripheral nervous system with diminished or reduced effects in the central nervous system. The compounds are particularly useful in treating disorders or conditions ameliorated by antagonism of peripheral 5-HT.sub.3 receptors. Moreover, side-effects attributable to antagonism of central 5-HT.sub.3 receptors can be lessened or reduced using the peripherally restricted compounds of the invention.