Cell genesis in the subgranular zone of the dentate gyrus of 2-month-old rabbits has been investigated. After incorporation of tritiated thymidine, electron microscopic autoradiography allowed description of the ultrastructure of the cells labelled and the progressive transformation of these cells into granular neurons to be followed. Quantitative evaluation of the time course of this transformation has been performed by light microscope autoradiography using 1-micrometer sections. Precursor cells, labelled initially with 3H-thymidine, were transformed after 5 days into early neuroblasts, these cells in turn giving rise to neurons some 8 days later. At the latest time period examined (42 days), 80% of the labelled cells were neurons; more than 10% remained as precursor cells. It is suspected that the latter may behave as reserve cells. Small numbers of glial cells, astrocytes, and microglia, scattered throughout the hilus of the dentate gyrus and the molecular layer, were found labelled, and it is possible that they arise from a different precusor pool. It is concluded that the subgranular zone functions as a secondary matrix for granule neurons of the dentate gyrus in young rabbits. These late-forming and apparently synaptically uncommitted neurons may be recruited during the development and refinement of postnatal behavioral substrates, by one or other of the dominant afferent systems.