By employing the autoradiographic method we have determined that the inferior olivary nucleus receives input from the reticular formation of the midbrain, pons and, probably, the medulla. The remarkable thing about such connections is that they are not diffuse, but targeted in large part on restricted portions of the caudal acessory nuclei. From our previous studies it is clear that some of the olivary regions receiving reticular input also receive projections from the cerebral cortex, the spinal cord and the dorsal column nuclei. In addition, the HRP technique reveals that these same olivary regions relay to parts of the spinal cerebellum, i.e. to restricted zones of the anterior lobe (present study). Taken together these observations suggest that some reticulo-olivo-cerebellar connections provide indirect routes through which cortical and spinal information gains access to the spinal cerebellum. Such circuits have also been suggested by the physiological literature. Of particular interest, however, was our finding that several reticular areas of the midbrain and pons project to the portion of the medial acessory nucleus which relays in turn to auditory-visual areas of the cerebellar vermis (declive, folium and tuber). It would appear that at least some reticulo-olivo-cerebellar circuits are significant in the organization of motor responses to visual and/or auditory stimuli.