OLFACTORY receptor neurons depolarize in response to odorant stimulation of their sensory cilia1-3. One transduction mechanism involves a G-protein-mediated increase in adenylate cyclase activity4-8, raising the internal cyclic AMP concentration to open a cyclic nucleotide-activated cation channel on the plasma membrane9-14. An influx of Ca2+ through this channel, which is permeable to both monovalent and divalent cations, triggers olfactory adaptation15. Previous work has indicated that at least art of this Ca2+ mediated adaptation resides in the channel itself15-17, but the mechanism remains unclear and controversial16-18. Here we use the cloned channel from rat19 expressed in a cell line and the native channel from rat olfactory receptor cells to show that Ca2+ reduces the apparent affinity of the channel for cAMP by up to 20-fold in the presence of calmodulin, an abundant protein in olfactory cilia20. This decrease in apparent affinity appears to involve a direct interaction between Ca2+-calmodulin and the channel, and it can reduce the activation of the channel by cAMP by up to a few hundred-fold, suggesting that it may be a key component of the Ca2+-triggered olfactory adaptation.