Cell proliferation and migration in the developing mouse olfactory bulb was studied by autoradiography. Animals were injected with thymidine-H3 at various developmental stages and killed one to 96 hours later. All neurons arise in the germinal zone surrounding the ventricle. Until the fourteenth day of gestation (E14) this zone consists only of a matrix (primitive ependymal) layer. From E14 to E17 both matrix and subependymal layers are found, and from E18 to maturity only the subependymal layer is present. The matrix layer produces the mitral cells and some tufted and granule cells; the subependymal layer gives origin to some tufted cells and most of the granule cells. Mitral cell neuroblasts migrate peripherally to reach the primitive mitral cell layer three days after final DNA synthesis. Tufted cell neuroblasts migrate through the developing mitral cell layer to reach their definitive locations; external granule cell neuroblasts migrate past both developing mitral and tufted cells. Neuroglia arise from scattered proliferating glioblasts, originally derived from the periventricular germinal zone. Time of origin and rate of migration of olfactory bulb neurons was checked against the histological development of the bulb. A good agreement corroborated the autoradiographic method used in this study and in previous studies on time of origin of neurons in the central nervous system.