Neuronal nicotinic acetylcholine receptors (AChRs) are composed of two types of subunits: ACh-binding (termed α2, α3, α4 ...) and structural (termed β2, β3, β4 ...). AChR subtypes composed of combinations of subunits of these two types encoded by several related genes are expressed in different parts of the nervous system, where they presumably serve different functional roles. Here we identify the ACh-binding subunit of the most prominent chicken brain AChR subtype by N-terminal amino acid sequence and show that it corresponds to the α4 gene. Previously we identified the structural subunit for this AChR subtype from chicken brain as β2 by N-terminal amino acid sequence. Thus, this identifies both genes which encode subunits of the major nicotinic AChR subtype in avian brains. By immunoprecipitation, immunohistochemistry, and northern blot analysis we show that α3 (or a very closely related sequence) is expressed at low levels in the brain and relatively high levels in the retina, while α4 is expressed at high levels in the brain and lower levels in the retina. This differential expression indicates that α3-containing 'ganglionic-type' AChRs may be an important AChR subtype in avian retina. © 1991.