A physiological model of a post-cochlear mechanism responsible for enhancing the phase-locked activity of auditory neurons is proposed, when excited by sinusoïdally amplitude-modulated (AM) tones. In the cochlear nuclei, chopper units respond to the summated low-frequency components of their peripheral inputs. This feature is due to the time constant of the excitatory postsynaptic potential. It is also the result of the specific time course of the cellular membrane potential during the refractory period. The physiologically plausible model is derived from Hodgkin–Huxley equations. AM tones and white noises were applied into the chopper model. In both experiments, the responses of the model showed a strong phase-locking for modulation frequencies between 100 and 300 Hz. The mean gain of the chopper model suggests that chopper units enhance the phase-locked AM responses that they receive as inputs from auditory-nerve fibers. These results agree with earlier electrophysiological studies.