Neuronal nicotinic acetylcholine receptors are members of a gene family of ligand-gated transmitter receptors that includes muscle nicotinic receptors, GABA receptors and glycine receptors. Several lines of evidence indicate that neuronal nicotinic receptors can be made up of only two subunits, an alpba (α) subunit which binds ligand, and a non-alpha (nα) or beta (β) subunit. The stoichiometry of each subunit in tbe functional receptor has been difficult to assess, bowever. Estimates or the molecular weight of neuronal nicotinic receptor macromolecules suggest that these receptors contain at least four subunits but probably not more than fiye. We have examined the subunit stoichiometry or the chick neuronal α/nα receptor by first using site-directed mutagenesis to create aubunits that confer different single cbannel properties on the receptor. Co-injection with wild-type and mutant subunits led to the appearance of receptors with wild-type, mutant and hybrid conductances. From the number of hybrid conductances, we could deduce the number or each subunit in the functional receptor.