The generation of an excitatory receptor current in mammalian olfactory sensory neurons (OSNs) involves the sequential activation of two distinct types of ion channels: cAMP-gated Ca²⁺-permeable cation channels and Ca²⁺-gated Cl^- channels, which conduct a depolarizing Cl^- efflux. This unusual transduction mechanism requires an outward-directed driving force for Cl^-, established by active accumulation of Cl^- within the lumen of the sensory cilia. We used two-photon fluorescence lifetime imaging microscopy of the Cl^--sensitive dye 6-methoxy-quinolyl acetoethyl ester to measure the intracellular Cl^- concentration in dendritic knobs of OSNs from mice and rats. We found a uniform intracellular Cl^- concentration in the range of 40-50 mm, which is indicative of active Cl^- accumulation. Functional assays and PCR experiments revealed that NKCC1-mediated Cl^- uptake through the apical membrane counteracts Cl^- depletion in the sensory cilia, and thus maintains the responsiveness of OSNs to odor stimulation. To permit Cl^- accumulation, OSNs avoid the “chloride switch”: they do not express KCC2, the main Cl^- extrusion cotransporter operating in neurons of the adult CNS. Cl^- accumulation provides OSNs with the driving force for the depolarizing Cl^- current that is the basis of the low-noise receptor current in these neurons.