The odorant-induced Ca(2+) increase inside the cilia of vertebrate olfactory sensory neurons controls both excitation and adaptation. The increase in the internal concentration of Ca(2+) in the cilia has recently been visualized directly and has been attributed to Ca(2+) entry through cAMP-gated channels. These recent results have made it possible to further characterize Ca(2+)'s activities in olfactory neurons. Ca(2+) exerts its excitatory role by directly activating Cl(-) channels. Given the unusually high concentration of ciliary Cl(-), Ca(2+)'s activation of Cl(-) channels causes an efflux of Cl(-) from the cilia, contributing high-gain and low-noise amplification to the olfactory neuron depolarization. Moreover, in combination with calmodulin, Ca(2+) mediates odorant adaptation by desensitizing cAMP-gated channels. The restoration of the Ca(2+) concentration to basal levels occurs via a Na(+)/Ca(2+) exchanger, which extrudes Ca(2+) from the olfactory cilia.