A Ca²⁺-activated Cl^- current constitutes a large part of the transduction current in olfactory sensory neurons. The binding of odorants to olfactory receptors in the cilia produces an increase in cAMP concentration, Ca²⁺ enters into the cilia through CNG channels and activates a Cl^- current. In intact mouse olfactory sensory neurons little is known about the kinetics of the Ca²⁺-activated Cl^- current. Here, we directly activated CNG channels by flash photolysis of caged cAMP or 8-Br-cAMP and measured the current response with the whole-cell voltage-clamp technique in mouse neurons. We measured multiphasic currents in the rising phase of the response at -50 mV. The current rising phase became monophasic in the absence of extracellular Ca²⁺, at +50 mV, or when most of the intracellular Cl^- was replaced by gluconate to shift the equilibrium potential for Cl^- to -50 mV. These results show that the second phase of the current in mouse intact neurons is due to a Cl^- current activated by Ca²⁺, similarly to previous results on isolated frog cilia. The percentage of the total saturating current carried by Cl^- was estimated in two ways: (1) by measuring the maximum secondary current and (2) by blocking the Cl^- channel with niflumic acid. We estimated that in the presence of 1 mM extracellular Ca²⁺ and in symmetrical Cl^- concentrations the Cl^- component can constitute up to 90% of the total current response. These data show how to unravel the CNG and Ca²⁺-activated Cl^- component of the current rising phase.