Arylpiperazines are nonselective agents that bind at 5-HT3 serotonin receptors with moderate to high affinity, whereas 1-phenylbiguanide is a low-affinity but more selective 5-HT3 agonist. In an attempt to enhance the affinity of the latter agent, and working with the assumption that similarities might exist between the binding of the two types of agents, we formulated structure-activity relationships for the binding of the arylpiperazines and then incorporated those substituents, leading to high affinity for the arylpiperazines, into 1-phenylbiguanide. A subsequent investigation examined the structure-activity relationships of the arylbiguanides and identified arylguanidines as a novel class of 5-HT3 ligands. Although curious similarities exist between the structure-activity relationships of the arylpiperazines, arylbiguanides, and arylguanidines, it cannot be concluded that all three series of compounds are binding in the same manner. Furthermore, upon investigating pairs of compounds in the three series, the arylpiperazines behaved as 5-HT3 antagonists (von Bezold-Jarisch assay) whereas the arylbiguanides and arylguanidines acted as 5-HT3 agonists.