Afferents to the fetal rat cerebellum have been studied in fixed tissue with the fluorescent tracer, 1,1'-dioctadecyl-3,3,3',3'tetramethylindocarbocyanine perchlorate (DiI). The dye was applied to the cerebellar anlage at ages from embryonic day (E) 12 to birth (P0). Central processes of vestibular ganglion cells were found to be the first identifiable afferents to the cerebellum, being present at least by E13 and perhaps as early as E12. Ipsilateral spinocerebellar fibres may be labelled from E15, vestibular nuclei (both ipsi- and contralateral) also from E15, while contralateral inferior olivary nuclei could not be retrogradely labelled until E17. Trigeminocerebellar neurons in the interpolaris subnucleus of the nucleus of the trigeminal spinal tract and neurons of the lateral reticular nucleus were not labelled until E22 and P0, respectively. Finally, contralateral pontine nuclei were retrogradely labelled from the cerebellum after birth.