Many projection systems within the peripheral and central nervous system are topographically organized, and it has become increasingly clear that interactions which occur during development determine the projection patterns these systems exhibit in the adult. The olivocerebellar system was chosen as a model system for this study of afferent pattern formation because it has several characteristics which lend themselves to a study of this type. Applications of horseradish peroxidase were made to both the cerebellar primordium and to the inferior olive of embryonic and neonatal mice using an in vitro perfusion system to support the tissue during the transport period. Fibers labeled after restricted olivary applications are limited to particular mediolateral regions of the cerebellum. Similarly, olivary cells retrogradely labeled after discrete cerebellar applications are restricted to particular olivary subdivisions. The results indicate that the olivocerebellar projection displays elements of topographic organization as early as E15 and that the pattern displayed is roughly comparable to that of the adult mammal. The observed trajectories of olivocerebellar fibers and their concomitant association with both Purkinje and cerebellar nuclear cells during embryonic development suggests a role for either or both cell types in the pattern formation process.