Early stages in chick neurogenesis were investigated with tritiated thymidine (3H-Tdr) autoradioraphy to determine the location and identity of the first neurons produced for the central nervous system. These cells have been shown to arise prior to neural tube closure (Sechrist, '75). Chicks were treated at selected intervals between 20 and 72 hours of incubation with 3H-Tdr in a modified pulse-labeling technique, and terminated on the 18th day of embryonic development (E18), when neuronal types could be determined. Some of the earliest neurons start their final DNA synthesis before 20 hours of incubation (head process, Hamburger-Hamilton stage 5). These are primarily medium-sized cells of the reticular formation in the medulla and at the diencephalic-mesencephalic junction, but also in the intermediate zone of the spinal cord. Motor neurons of the brainstem and spinal cord begin to appear next, after 26-28 hours incubation; the first sensory neurons arise after 32 hours. Other workers (Ramon y Cajal, '60; Tello, '23; Windle and Austin, '36) found that neurons of the reticular formation were the first to differentiate neurofibrils, during the latter part of E2, indicating that fibrillogenesis in these cells begin about 24 hours after the initial cessation of DNA replication.