Corticonuclear (CN) and nucleocortical (NC) projections of the cerebellum were studied in rats by using antero- and retrograde transport of wheatgerm agglutinin-conjugated horseradish peroxidase (WGA-HRP). Following minute injections of WGA-HRP in different folia for vermal lobules VI-VIII, a rostrocaudal topography was observed in the CN projection; lobules VIa and VIb, c projected to distinctly separate areas in the ipsilateral fastigial nucleus (FN): VIa projected to the ventral part of the medial subdivision of the FN (FNm), whereas VIb, c projected to the dorsocaudal part of the caudomedial subdivision of the FN (FNcm) and to the caudal FNm. The terminals from lobules VIb, c and VII occupied partially overlapping areas in the caudal part of the ipsilateral FNcm and FNm. A few terminal fibers were also seen in the dorsolateral protuberance of the FN (dlp). The fibers from vermal lobule VIII terminated in the ipsilateral rostral FNcm and in the FNm. The latter terminal area was divided into two parts: a smaller dorsal and a larger ventral part that in turn overlapped almost completely with the terminal area of fibers from lobule VIa. Retrogradely labeled fastigial neurons were found in the areas of the FN where Purkinje-cell (P-cell) axons from the respective injection sites terminated. In addition, retrogradely labeled fastigial neurons were scattered in areas outside the P-cell terminal fields on the injection side and in the contralateral FN.