Previous studies have demonstrated that relatively low levels of alpha4beta2 neuronal nicotinic acetylcholine receptors (nAChRs) are expressed on the cell surface of transfected mammalian cell lines but that surface expression levels can be dramatically up-regulated by co-expression of these subunits with chimeric subunits containing the N-terminal portion of the neuronal nAChR alpha4 or beta2 subunits together with the C-terminal domain of the 5-HT(3A) subunit. Recent work has also suggested that the nAChR alpha4 subunit can co-assemble in a "promiscuous" manner with the serotonin receptor 5-HT(3A) subunit to form functional hybrid receptors. In this study we have examined whether co-assembly of either alpha4 or beta2 with 5-HT(3A) itself (rather than with the alpha4/5-HT(3A) or beta2/5-HT(3A) subunit chimeras) can also facilitate cell surface expression of alpha4 and beta2 subunits in transfected mammalian cells. Evidence has been obtained by immunoprecipitation, cell-surface antibody binding and radioligand binding which indicates that the 5-HT(3A) can co-assemble with both the alpha4 and beta2 nAChR subunits. We conclude, however, that co-assembly of 5-HT(3A) with either alpha4 or beta2 does not result in efficient cell surface expression of the nAChR subunits and that co-assembled hybrid (nAChR subunit + 5-HT(3)R subunit) receptor complexes are largely retained within the cell.