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UNLIKE most neurones, the dentate granule cells continue to be produced in adults. Recently our study has demonstrated that a highly polysialylated neural cell adhesion molecule (NCAM-H) is a reliable molecular marker for newly generated and developing dentate granule cells. Here we examined age-related changes in the number of newly generated and developing granule cells. In rats injected with 5-bromo-2'-deoxyuridine (BrdU), a thymidine analogue, at 35 days to 18 months of age, double immunohistochemistry for BrdU and NCAM-H showed that the numbers of NCAM-H-positive and BrdU-/NCAM-H-positive granule cells gradually decreased over more than one year, but they were still detected in 18-month-old rats. These findings indicate that newly formed and developing granule cells occur in the dentate gyrus of young and aged rats, but the number of these cells decreases with age.