The reversible binding of(14)C-decamethonium (Deca) to excitable microsacs prepared from the electric tissue ofElectrophorus electricus is followed by an ultracentrifugal assay. α-Bungarotoxin, a snake venom toxin, blocks irreversibly the binding of(14)C-Deca. The displacement is partial. The fraction of(14)C-Deca displaced by α-bungarotoxin corresponds to molecules of Deca bound to the cholinergic receptor site, whereas the fraction of(14)C-Deca bound in the presence of α-bungarotoxin corresponds to molecules bound to the catalytic site of acetylcholinesterase (AcChE). The total number of cholinergic receptor sites is found to be close but not identical to the total number of catalytic sites of AcChE.On the same preparation of microsacs, the binding of(14)C-Deca and the permeability response corresponding to a given concentration of Deca are measured as a function of increased concentration of Deca. The dose-response curve and the binding curve superimpose almost exactly; in other words, the "apparent" affinity of Deca coincides with its "real" affinity. Displacement of(14)C-Deca byd-tubocurarine gives an "apparent" affinity ford-tubocurarine which coincides as well with its "real" affinity.The transport properties of the ionophore controlled by one Deca binding site are estimated.