Nucleus magnocellularis (NM) and nucleus laminaris (NL) are, respectively, second- and third-order auditory nuclei in the chicken brain stem. In this report the morphogenesis of these nuclei is examined. The times of origin of the cells of these nuclei were studied with 3H-thymidine autoradiography. The number of cells in each nucleus and their rostro-caudal distribution within each nucleus was determined in Nissl-stained sections at 9, 11, 13, 15, 17, 19, and 21 days of incubation. For the above ages the volumes of NM and NL were also calculated planimetrically and the rostro-caudal distribution of volume within each nucleus was studied.