5-HT(3) receptors are widely distributed in the CNS and PNS where they participate in a variety of physiological processes. Native 5-HT(3) receptors in the CNS display functional and pharmacological heterogeneity, suggesting the existence of multiple receptor subunits. However, recent evidence suggests that of the two known subunits only the 5-HT(3A) receptor subunit (and not the 5-HT(3B) receptor subunit) is functionally present in the CNS. The molecular makeup of the 5-HT(3) receptor therefore remains an open question.