We have found that the acetylcholine receptor (AChR) of Torpedo californica is phosphorylated and dephosphorylated in situ by a membrane-bound protein kinase and phosphatase . There is increasing evidence that other neurotransmitters , light [3-6], polypeptide hormones , and growth factors [8-12] also may act by regulating the level of phosphorylation of membrane proteins. These observations suggest that membrane protein phosphorylation may be a general regulatory mechanism affecting the response of cells to exogenous metabolic and physical signals. To understand the role of membrane protein phosphorylation, we have chosen to study acetylcholine receptor-enriched membranes purified from the electric organ of T californica. This organ is an ideal model system for such studies since it is a rich source of the acetylcholine receptor. Membranes can be purified from the electric organ that are enriched in the AChR and that show cholinergic agonist-dependent changes in cation flux . Moreover, the AChR from T californica has been purified [15[, biochemically characterized , and used to generate specific antibodies . Receptor, enriched membranes contain only a few other proteins that are closely associated with the receptor in the postsynaptic membrane. Such associated proteins may play a critical role in regulating the function of the AChR in the postsynaptic membrane. We have taken advantage of these conditions to study phosphorylation of the membrane-bound AChR in this well-defined, homogeneous system.