The primary structures of nicotinic acetylcholine receptors and sodium channels from fish and mammals have been elucidated by cloning and sequencing the cDNAs. The structural basis for the operation of the acetylcholine receptor has been investigated by analysing the functional properties of mutant receptors produced by expression of the cDNAs altered by site-directed mutagenesis as well as those of hybrid receptors composed of subunits from different species. Structural features characteristic of the sodium channel are discussed in terms of its function. The evolution of the ionic channels is also discussed.