Series of clicks, with a standard number and greatly varying by the length of interstimuli intervals in each series, were systematically presented to dogs without reinforcement. Evoked potentials (EP) in the cortical auditory zone were recorded. A typical U-shaped distribution of amplitude of averaged EP was found in the series in the course of repeated presentations; a maximal amplitude was observed in response to the 1st and last clicks, and a minimal, in the middle of the series. Such a regularity was previously found for the action of standard series with a constant interstimulus interval. An estimation may take place in the dog CNS of the number of signals used in a series, whose mechanism is not necessarily based on the time factor. The ability to estimate the number of signals in the series may be regarded as an elementary model of counting in the dog CNS.