After a bilateral ablation of the rat auditory cortex, differentiation of a tonal signal from an amplitude-modulated stimulus with 50 c/s modulation frequency is preserved, while ablation of the posterior colliculi completely abolishes differentiation of these signals, without subsequent restoration in six months after the operation. At the same time the same rats are able to elaborate a differentiation of a tonal signal from an amplitude-modulated signal with a 500 c/s modulation frequency. The results obtained show that the posterior colliculi are at the level of the auditory system, where the coding of signals with a modulation frequency of 50 c/s amplitude is accomplished, whereas stimuli with a higher frequency of amplitude modulation are coded at a lower level of the auditory system.