The single unit activity of cochlear nuclei in Verspertilionidae are studied by means of ultrasonic pure tonal and frequency-modulated stimuli. Most neurons (85%) had tonic reaction to combination tones f₂-f₁ and 2f₁-f₂ (f₁, pulse frequency of the first tone, f₂ pulse frequency of the second tone) as a result of the nonlinearity in the auditory system. The reaction to the combination tones depends upon pulse frequency and sound pressure level of each of the presented stimuli. Tonic reaction appeared in the case when the combination tone was equal to the best frequency of the neuron. The question about the role of the nonlinearity of the auditory system in the resolution of the complex signals is discussed.