The olivocerebellar projection has been studied in the rat using the horseradish peroxidase (HRP) retrograde tracing technique. All regions of the cerebellum, except for the flocculonodular lobe, were injected with small volumes of HRP. The overall organization of the rat olivocerebellar projection is similar in all major respects to that found in other mammals. For example, in the rat Crus 1 and Crus 2 receive afferents from the principal olive, medial part of the rostral dorsal olive, the rostral medial olive, the dorsal cap and the ventrolateral outgrowth which agrees with studies in the cat. The most notable difference between the present study and studies in other mammals is the finding of a projection in the rat from the dorsomedial cell column to all regions of the cerebellar hemispheres, except for the paraflocculus.