In olfactory receptor neurons, odorants elicit excitatory and inhibitory responses, associated to an increase and to a decrease in the action potential frequency, respectively. The inhibitory response involves the opening of cyclic nucleotide gated channels via the cyclic AMP cascade that leads to activation of Ca²⁺-dependent K⁺ (K_Ca) channels. Patch-clamp studies performed in our laboratory demonstrated the presence of low, intermediate and high conductance K_Ca channels in the olfactory cilia (Delgado et al., 2003). In the present study, we carried out the biophysical and pharmacological characterization of these ciliary K_Ca channels. This was addressed by studying the K_Ca channels from a purified rat olfactory cilia membrane fraction, incorporated into planar phospholipid bilayers. Olfactory tissue was obtained from adult rats sacrificed with anaesthesia overdose (pentobarbitone 60 mg/Kg), according with the guidelines of the Ethics Committee of the University of Chile. Data are expressed as mean ± S. D. Low (S), intermediate (I) and high (B) conductance K_Ca channels, were incorporated into the bilayers, in symmetrical 100 mM K⁺. The SK_Ca channel showed a 16.3 ± 0.7 pS (n = 6) conductance, insensitivity to voltage, and sensitivity to Ca²⁺ (K_0.5 = 57.1 ± 2.1 μM, n = 3) and to apamin. We identified two different IK_Ca channels. One presented two open conductance levels, of 30.3 ± 1.4 pS and 50.5 ± 1.9 pS (n = 7), was sensitive to Ca²⁺ (K_0.5 = 59.8 ± 0.9 μM, n = 2), clotrimazole and insensitive to voltage and to charybdotoxin. The other IK_Ca channel showed an open conductance of 59.6 ± 1.4 pS (n = 5) and was sensitive to clotrimazole. We also identified a BK_Ca channel with a 210.4 ± 5.8 pS (n = 12) conductance, Ca²⁺ dependent (K_0.5 = 63.4 ±2.4 μM, n = 8) and sensitive to iberiotoxin and charybdotoxin. The SK_Ca, the BK_Ca and one of the IK_Ca channels characterized on bilayers, were very similar to the K_Ca channels, recorded by patch-clamp, regarding their unitary conductance, calcium dependence and kinetics, suggesting that they correspond to the same K_Ca channels. The presence of the SK_Ca (SK3) and the BK_Ca channels in the ciliary fraction was confirmed by Western blotting. On the other hand, the voltage-dependent Na⁺ channel was not detected, indicating the high purity of the ciliary membranes. Our biophysical and pharmacological characterization of the ciliary K_Ca channels in bilayers would allow a better understanding of their contribution to the K_Ca current in the inhibitory response in olfactory neurons.