The neuronal nicotinic alpha 7 (nAChR) and 5-hydroxytryptamine (5HT3) receptors are ligand-gated ion channels with a homologous topological organization and have activation and desensitization reactions in common. Yet these homo-oligomeric receptors differ in the pharmacology of their binding sites for agonists and competitive antagonists, and in their sensitivity to Ca2+ ions. The alpha 7 channel is highly permeable to Ca2+ ions and external Ca2+ ions potentiate, in an allosteric manner, the permeability response to acetylcholine, as shown for other neuronal nAChRs. The 5HT3 channel, in contrast, is not permeable to Ca2+ ions, but blocked by them. To assign these properties to delimited domains of the primary structure, we constructed several recombinant chimaeric alpha 7-5HT3 receptors. We report here that one of the constructs expresses a functional receptor that contains the serotonergic channel still blocked by Ca2+ ions, but is activated by nicotinic ligands and potentiated by external Ca2+ ions.