Cholinergic nicotinic receptors have been investigated in the ciliary ganglion of adult chick by means of [ 125I]α-bungarotoxin binding to whole and homogenized ganglia. The behaviour of receptors has been compared in vivo (after simultaneous pre- and postganglionic axotomy) and in vitro (ganglia in organ culture). A decrease in receptor number was found both in vivo and in vitro, although the results differed quantitatively in the two cases. The loss of receptors was more evident in whole than in homogenized ganglia, suggesting that mechanisms of receptor degradation and masking may occur simultaneously. Additional experiments in vitro indicate that the behaviour of the receptors is influenced by inhibitors of energy metabolism (dinitrophenol plus iodoacetic acid, sodium cyanide) but not by an inhibitor of protein synthesis (cycloheximide). The receptor modifications in vitro are accompanied by a decrease of other cholinergic markers, such as acetylcholinesterase and cholinacetyltransferase activities. Morphological investigations of organ-cultured ganglia indicate that the biochemical modifications described above are not due to significant degenerative changes of ganglionic neurons. The results indicate that the α-bungarotoxin receptors behave rather similarly in vivo and, under our conditions, in vitro and suggest a dependence of these receptors on peripheral signals. These signals would appear to be mediated by energy-dependent processes without the synthesis of proteins being required.