Real-time examination of Dil-labeled, immature granule cells in cerebellar slice preparations reveals several temporal and cytological aspects of neuronal migration that have not been observed in previous in vivo or in vitro systems. Using confocal microscopy we have obtained evidence that rates of cell movement depend critically on the age of the cerebellum. Although there were considerable variations in the speed of individual cells, the average rate of cell migration increased systematically from 9.6 +/- 3.0 microns/hr in cerebella from 7-d-old mice to 18.0 +/- 2.9 microns/hr in cerebella from 13-d-old mice. Consequently, granule cells traversed the developing molecular layer within a relatively constant time period despite the doubling in width of the molecular layer during the second week of postnatal life. Granule cell movement was characterized by alternations of short stationary phases with movement in a forward or backward direction. The net displacement of a cell depended on the duration and frequency of these phases as well as on the speed of movement. Changes in the relative position of Dil crystals attached to the surface of granule cells suggested the existence of a complex topographical flow of plasma membrane during migration. Although a large portion of the plasma membrane seemed to move in register with the nucleus and surrounding cytoplasm, new membrane appeared to be incorporated primarily at the leading process. However, the pattern of membrane flow at the interface between migrating neurons and Bergmann glial fibers could not be determined, since these sites could not be labeled by Dil crystals. The present results are in harmony with the concept that multiple cellular/molecular mechanisms may be engaged in granule cell migration.