Adult Old World primates, including macaque monkeys, produce new hippocampal neurons, as demonstrated by the presence of bromodeoxyuridine (BrdUrd)-labeled cells in the dentate gyrus and other brain regions. This study used BrdUrd injections and immunohistochemical markers to identify new neurons in adult monkeys of different ages. The results showed that young and middle-aged monkeys had a substantial number of BrdUrd-labeled cells with characteristics of immature and mature neurons. In older monkeys, the number of new neurons was lower but still present. These findings indicate that adult Old World monkeys can generate new hippocampal neurons, suggesting that macaque monkeys may be a useful model for studying adult neurogenesis. The study also found that new neurons in the dentate gyrus expressed markers of immature and mature granule neurons, including TOAD-64, calbindin, NSE, and NeuN. These neurons were located in the subgranular zone and the hilus, and their presence supports the idea that new neurons are continuously generated in the adult brain. However, previous studies using ³H-thymidine autoradiography had reported no neurogenesis in adult Old World monkeys, which may be due to long survival times that led to the loss of labeled cells. The findings suggest that the hippocampal formation may play a role in learning and memory, and that the production of new neurons may be influenced by environmental factors and stress. The study also highlights the importance of controlling glucocorticoid levels to maintain hippocampal neurogenesis in aging brains. Overall, the results indicate that adult neurogenesis is a common phenomenon in many mammalian species, including humans, and that the hippocampal formation may be involved in the formation of new memories.Adult Old World primates, including macaque monkeys, produce new hippocampal neurons, as demonstrated by the presence of bromodeoxyuridine (BrdUrd)-labeled cells in the dentate gyrus and other brain regions. This study used BrdUrd injections and immunohistochemical markers to identify new neurons in adult monkeys of different ages. The results showed that young and middle-aged monkeys had a substantial number of BrdUrd-labeled cells with characteristics of immature and mature neurons. In older monkeys, the number of new neurons was lower but still present. These findings indicate that adult Old World monkeys can generate new hippocampal neurons, suggesting that macaque monkeys may be a useful model for studying adult neurogenesis. The study also found that new neurons in the dentate gyrus expressed markers of immature and mature granule neurons, including TOAD-64, calbindin, NSE, and NeuN. These neurons were located in the subgranular zone and the hilus, and their presence supports the idea that new neurons are continuously generated in the adult brain. However, previous studies using ³H-thymidine autoradiography had reported no neurogenesis in adult Old World monkeys, which may be due to long survival times that led to the loss of labeled cells. The findings suggest that the hippocampal formation may play a role in learning and memory, and that the production of new neurons may be influenced by environmental factors and stress. The study also highlights the importance of controlling glucocorticoid levels to maintain hippocampal neurogenesis in aging brains. Overall, the results indicate that adult neurogenesis is a common phenomenon in many mammalian species, including humans, and that the hippocampal formation may be involved in the formation of new memories.