The odorant-induced Ca²⁺ increase inside the cilia of vertebrate olfactory sensory neurons controls both excitation and adaptation. The increase in the internal concentration of Ca²⁺ in the cilia has recently been visualized directly and has been attributed to Ca²⁺ entry through cAMP-gated channels. These recent results have made it possible to further characterize Ca²⁺'s activities in olfactory neurons. Ca²⁺ exerts its excitatory role by directly activating Cl⁻ channels. Given the unusually high concentration of ciliary Cl⁻, Ca²⁺'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, Ca²⁺ mediates odorant adaptation by desensitizing cAMP-gated channels. The restoration of the Ca²⁺ concentration to basal levels occurs via a Na+/Ca²⁺ exchanger, which extrudes Ca²⁺ from the olfactory cilia.