Cyclic nucleotide-gated (CNG) ion channels are multimeric proteins that activate in response to the binding of cyclic nucleotide to intracellular domains. Here, an intramolecular protein-protein interaction between the amino-terminal domain and the carboxylterminal ligand-binding domain of the rat olfactory CNG channel was shown to exert an autoexcitatory effect on channel activation. Calcium-calmodulin, which modulates CNG channel activity during odorant adaptation, blocked this interaction. A specific deletion within the amino-terminal domain disrupted the interdomain interaction in vitro and altered the gating properties and calmodulin sensitivity of expressed channels. Thus, the amino-terminal domain may promote channel opening by directly interacting with the carboxyl-terminal gating machinery; calmodulin regulates channel activity by targeting this interaction.