The dorsal cap and ventrolateral outgrowth of the inferior olive are involved in the control of eye movements. The caudal dorsal cap is predominantly involved in the horizontal optokinetic reflex; it receives most of its GABAergic input from the nucleus prepositus hypoglossi. In the present study, we determined the source of a major inhibitory input to the rostral dorsal cap and the ventrolateral outgrowth, which are the olivary subnuclei mainly involved in the "vertical" optokinetic reflexes. We studied these subnuclei in the rabbit with the use of retrograde tracing of horseradish peroxidase and anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase combined with postembedding immunocytochemistry. The ventral dentate nucleus of the cerebellum and dorsal group y project contralaterally to the rostral dorsal cap and ventrolateral outgrowth; this projection is entirely GABAergic. The terminals of this input form predominantly symmetric synapses with extraglomerular and intraglomerular dendrites; the remaining terminals are axosomatic. In addition, the dorsal cap and ventrolateral outgrowth contain significantly more crest synapses than any other olivary subnucleus. The terminals that form these crest synapses are derived from dorsal group y and/or the ventral dentate nucleus. None of the terminals in the dorsal cap or ventrolateral outgrowth was glycinergic.