The organization of the olivocerebellar and corticonuclear relations for vermal lobules IX and X and the flocculus has been studied in the rat by using microinjections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). This axonal tracer allowed us to study simultaneously the olivocortical connections (revealed by retrograde transport) and corticonuclear connections (revealed by anterograde transport) from a single injection in the cerebellar cortex. The results indicate that four modules can be distinguished, each of which consists of a region of cerebellar cortex receiving afferents from a single small region of the inferior olive (IO) and sending efferents to one or several portions of the cerebellar nuclei and/or vestibular nuclei. The first module includes a medial part of lobule X as well as all the flocculus. It receives afferents from the dorsal cap (dc) and sends efferents to the small cell (s) zone of the dentate nucleus as well as to the medial vestibular (VM) nucleus and subnucleus y. The second module includes a medial parasagittal region located in lobules IX and X. It receives afferents from the ventrolateral outgrowth (vlo) and/or beta nucleus (vlo + beta nucleus) and sends efferents principally to the ventrolateral part of fastigial nucleus and to the superior vestibular (VS), inferior vestibular (VI), and VM nuclei. The third module includes a lateral parasagittal region in lobules IX and X. It receives afferents from the dorsomedial cell column (dmcc) of IO and sends efferents principally to the interpositus nucleus and subnucleus y. The fourth module includes the most lateral part of lobules IX and Xa. It receives afferents from the principal olive (PO) and sends efferents to the s zone of the dentate nucleus. These results are comparable to those obtained in the cat although a few differences are discussed.