Climbing fiber projections from the inferior olive to the nodulus of the cerebellum were studied by means of the retrograde transport of horseradish peroxidase in the cat. Following large and small injections into various parts of the nodulus, the distribution of labeled cells in the inferior olive was investigated. The findings indicate the existence of five longitudinal zones extending throughout the dorsal and ventral nodulus: (1) the caudal part of the nucleus beta projects to a most medially located zone (caudal beta zone) with a width of about 0.6 mm; (2) the rostral part of the nucleus beta projects to a zone located at 0.6-1.2 mm from the midline (rostral beta zone); (3) the caudal part of the dorsal cap (dc) projects to a zone located in the intermediate part of the nodulus at 1.2-1.8 mm from the midline (caudal dc zone); (4) the ventrolateral outgrowth (vlo) of the dc and the rostral part of the dc project to a zone at 0.3-0.9 mm from the lateral edge of the nodulus (rostral dc and vlo zone); and (5) finally the dorsomedial cell column (dmcc) projects to the most lateral zone (dmcc zone).