<p>Our previous study showed that oculomotor internuclear neurons project to precerebellar populations lying in the paramedian tract cell groups, in addition to projecting to the abducens nucleus. Consequently, we posited that the abducens internuclear neurons might have similar projections, beyond their well-recognized projection to the oculomotor nucleus. With this in mind, we examined macaque monkey cases with abducens nucleus injections of anterograde tracers. As with oculomotor internuclear neurons, we found terminal fields in paramedian tract cell groups. These abducens nucleus injections, also involved precerebellar projection cells located in the infragenual and supragenual regions. This was evidenced by the presence of labeled terminal arbors within the flocculus. Unexpectedly, we also noted previously undescribed labeled terminal arbors within the fastigial nucleus. Consequently, we tested whether the paramedian tract cell groups also project to this deep cerebellar nucleus. Injections of tracer into the fastigial nucleus retrogradely labeled neurons within the infragenual and supragenual regions, and in other components of the paramedian tract cell groups. Taken together with the previous study, we have strong neuroanatomical evidence suggesting that both the oculomotor and abducens internuclear projections provide a horizontal gaze efference copy signal to the cerebellum via the precerebellar populations located in the paramedian tract cell groups. In addition to supplying these signals to the flocculus, they also supply input to the fastigial nucleus. Thus, the internuclear projections may allow the fastigial nucleus and flocculus to appropriately modulate both conjugate and vergence eye movements by comparing the output signal to the actual movements produced.</p>