These neuroanatomical studies utilize 3H-thymidine autoradiography, cell counting methods, and bromodeoxyridine immunohistochemistry to study neuronogenesis in the adult dentate gyrus. We have found that the dentate granule cells which are generated in adult rats can extend axons through an apparently mature neuropil, and for appreciable distances. To determine if new neurons are added to the adult primate brain, we are studying neuronogenesis in the dentate gyrus of pygmy marmosets. And to facilitate the study of adult mammalian neuronogenesis we are utilizing immunohistochemical detection of the thymidine analog, bromodeoxyuridine.