Granule neurons of the rat dentate gyrus are born in adulthood as well as during development. Apoptotic cell death also occurs normally in this population throughout the life of the rat. Removal of adrenal steroids results in both increased production and increased degeneration of dentate gyrus granule cells. In order to determine whether the age of a cell affects its response to adrenalectomy (ADX), the numbers of dentate gyrus cells of different ages were assessed following ADX or sham operation. Older cells, i.e., those labeled with the thymidine analog bromodeoxyuridine (BrdU) on postnatal day (P) 6, were reduced in number following ADX on P60, and some had the morphologic characteristics of degenerating cells, indicating that significant numbers of mature cells die in response to ADX. In contrast, the number of younger cells, labeled with 3H-thymidine or BrdU in adulthood, 24 hours or 2 weeks before ADX, did not decrease, suggesting that these less mature cells do not die following ADX. An increase in the number of cells that are immunoreactive for proliferating cell nuclear antigen, a marker of dividing or recently mitotic cells, indicates that immature dentate gyrus cells divide following ADX. These results suggest that following ADX, mature cells born during the 1st postnatal week die, whereas immature cells divide.