The traditional view of the external granular layer of the cerebellar cortex giving rise to interneurons has been challenged by recent studies with quail-chick chimeras. To clarify the time and site of origins of interneurons, a retrovirus carrying the beta-galactosidase gene was injected into the deep cerebellar tissue or external granular layer of postnatal day 4/5 rats to label dividing progenitors. After deep cerebellar tissue injection, unipolar cells were found initially in white matter at 2 days postinjection and subsequently in the internal granule and molecular layers 4-6 days postinjection. Morphologically defined basket, stellate, and Golgi neurons were clearly identified by 20 days postinjection. In contrast, retroviral labeling of cells in the external granular layer produced only granule neurons in the internal granule layer. Thus, dividing progenitors in the cerebellar white matter migrate through the white matter into the cortex before differentiating into a variety of cortical interneurons.