Proton-activated ion channels of the ASIC 1a subtype in pheochromocytoma PC12 cells were inactivated under the long-lasting influence of an acidic milieu (pH 6.0) because of desensitization developing in a monoexponential mode with τ = 1.7 ± 0.3 sec. The recovery of currents after desensitization was also monoexponential, with τ = 4.9 ± 0.8 sec. Approximation of the dose−effect curve by the Hill equation showed that activation of a channel of this type needs, most probably, binding of three hydrogen ions.