Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into the individual vestibular nuclei of the rat to study their efferent connections. One of the major differences between the connections of these nuclei was found at the level of the mesencephalon: the eye-moving cranial nerve nuclei received the densest projection from the superior vestibular nucleus (SVN). In the present electron microscopic study, we have found that terminals of SVN origin established symmetric synaptic contacts in the oculomotor nucleus. More than two-thirds of PHA-L-labeled boutons terminated on dendrites, the rest of them established axosomatic contacts. Most of the labeled terminals were GABA-positive, supporting the results of previous physiological experiments, which showed inhibitory effects. In the mesencephalon, the other termination area was found in the red nucleus. The PHA-L-labeled boutons of SVN origin were in close contact with the perikarya and proximal dendrites of the magnocellular part of the red nucleus. The types of synaptic contacts and distribution of terminals of SVN origin were similar to those found in the oculomotor nucleus. Our results indicate that the SVN can modify the activity of the cerebellorubral and corticorubral pathways, exerting inhibitory action on the neurons of the red nucleus.