Chick ciliary ganglion neurons have nicotinic acetylcholine receptors (AChRs) that mediate synaptic transmission through the ganglion. A cAMP- dependent process has previously been shown to enhance the ACh response of the neurons 2- to 3-fold without requiring the synthesis of new receptors. We show here that the receptors can be phosphorylated in situ by a cAMP-dependent process. The phosphorylation occurs predominantly on components of 50 and 58 kDa. Both derive from putative ligand-binding alpha 3 subunits, with the smaller phosphorylated species probably representing a degradation product of the larger. The increase in receptor phosphorylation caused by incubating the neurons with a cAMP analog parallels the increase observed in the ACh response, with respect to both time course and relative extent. The phosphorylation of ciliary ganglion AChRs differs from that reported for electric organ AChRs, which occurs primarily on the non-ligand- binding gamma and delta subunits and increases the rate of agonist- induced receptor desensitization.