The role of primary afferent innervation in the maintenance of neurons in the mammalian auditory system was assessed by performing unilateral removals of the cochlea in neonatal and mature ferrets of known birth dates. Removals were performed under steroid anesthesia and resulted in the complete destruction of the organ of Corti and the loss of at least 80% of type I spiral ganglion neurons. Four main age groups [postnatal days (P)5, P24, P90, and P180] were used. Additional animals received no surgery, partial removals, or complete removals at older ages. Three months after the cochlear removal the animals were reanesthetized and perfused. The brainstem and the temporal bones were wax-embedded, frontally sectioned, and Nissl-stained. Sections of the right and left cochlear nuclei were compared quantitatively. Removal of the cochlea at P5 resulted in the loss of more than 50% of large (nongranular) neurons throughout the ipsilateral cochlear nucleus. Lesions at older ages did not produce any neuron loss. The size of the remaining neurons was reduced by 10–15% in all age groups. Partial lesions at P5 produced a graded response in the cochlear nucleus that was related to the extent of the lesion. The developmental sensitive period for the effects of cochlear removal on the ferret cochlear nucleus is therefore over before the age (P28–P30) at which the animal begins to hear. The present result differs markedly from the chicken, in which the sensitive period for removal of the cochlea persists for at least 2 months after the onset of hearing.