The binding of [125I]alpha-bungarotoxin (ABTX) to homogenates of ciliary ganglia and irises from embryonic and posthatching chickens has been examined. Specific, high-affinity binding was found in both tissues [KD (iris) equals 2.5 nM; KD (ganglion) equals 2.7 nM]. Binding is saturated above 10 nM toxin concentration and is inhibited by low concentrations of the nicotinic antagonist d-tubocurarine. The binding may be associated with a nicotinic cholinergic receptor in both tissues. The amount of binding in the iris begins to increase soon after functional innervation is first observed, at 12 days of incubation (d.i.), and continues to increase up to four months after hatching (a.h.), the oldest age tested. In contrast, ABTX binding in the ciliary ganglion increases fourfold between 7 and 11 d.i., after which the amount of binding remains unchanged up to four months a.h. When compared to the development of choline acetyltransferase (ChAc) and acetylcholinesterase (AChE) activities in the ganglion and iris, ABTX binding follows a pattern similar to that of AChE activity. The largest increases in ChAc activity occur later than those of the postsynaptic markers. After 16 d.i. there are approximately 3 x 10(6) toxin molecules bound per neuron in the ciliary ganglion.