Astrocytes in the central nervous system (CNS) respond to injury and disease by proliferating and extending processes. The intermediate filament protein of astrocytes, glial fibrillary acidic protein (GFAP) also increases in astrocytes. These cells are called "reactive astrocytes" and are thought to play a role in CNS repair. We have previously demonstrated rapid increases (< 6 hours) in GFAP-immunoreactive and silver-impregnated glial processes in the chick cochlear nucleus, nucleus magnocellularis (NM), following cochlea removal or activity blockade of the eighth nerve. It was not known whether these changes were the result of glial proliferation, glial hypertrophy, or both. The present study examined the time course of astrocyte proliferation in NM following cochlea removal. Postnatal chicks received unilateral cochlea removal and survived for 6, 12, 18, 24, 36, 48, and 72 hours. Bromodeoxyuridine was used to label proliferating cells. The volume and number of labeled cells in NM was calculated for both the experimental and control sides of the brains for experimental animals was well as for unoperated control animals. A subset of astrocytes continuously divide in the normal posthatch chick brainstem. The percentage of labeled nuclei increases within NM 36 hours following cochlea removal and is robust by 48 hours. This increase is due to astrocyte proliferation within, rather than migration to, NM. These results indicate that rapid increases in GFAP following reduced activity are independent of cell proliferation. The time course of astrocyte proliferation suggests that cellular degeneration within the nucleus may play a role in upregulating astrocyte proliferation.