At the vertebrate neuromuscular junction and in the electroplax of eel and electric fish, the nicotinic alpha-bungarotoxin site and the nicotinic receptor involved in synaptic transmission are very tightly coupled and, indeed, appear to be the same molecular component. On the other hand, the nature of the relationship between the nicotinic receptor mediating synaptic events and the nicotinic alpha-bungarotoxin binding site in nervous tissue has been a matter of controversy over the last few years. Experimental studies have been accumulating which suggest that in many neuronal tissues these two components are distinct molecular entities with their own unique regulation. However, it also appears that in other nervous tissues, possibly in species lower on the evolutionary scale, the toxin binding site is part of the nicotinic receptor. An evaluation of all available evidence would point to the conclusion that, in neuronal tissues, the nicotinic acetylcholine receptor involved in synaptic events and the nicotinic alpha-bungarotoxin site can exist both in a tightly coupled form and one in which the two sites are mutually distinct. The possible physiological significance of the nicotinic alpha-bungarotoxin site is discussed in light of current experimental data. Evidence is available which may imply that the alpha-toxin site, whether it is present as a distinct entity or in association with the nicotinic acetylcholine receptor, is involved in trophic or growth related activities, as well as in other cellular functions. The possibility of an endogenous ligand for the nicotinic alpha-bungarotoxin site is also discussed.