The times of origin of neurons in sensorimotor pathways of the marsupial brush-tailed possum were determined with 3H-thymidine autoradiography. A series of 20 possums were injected with 3H-thymidine from postnatal days 5-95 and were normally allowed to survive until the brain cytoarchitecture was mature. Brain stem and spinal sensorimotor regions were not labelled in our study and presumably form before birth in order to enable the newborn young to make the journey from the birth canal to the pouch on its own. Neurogenesis in thalamic sensorimotor nuclei probably begins about the time of birth and continues into the 2nd week of postnatal life. Formation of neurons in the sensorimotor cortex and the basal ganglia occurs during the first 2 months of postnatal life and in the cerebellum during the first 3 months. This protracted postnatal development of telencephalic and cerebellar sensorimotor regions offers great advantages for developmental studies.