OBJECTIVE To determine whether there are direct projections from the nucleus retroambiguus to larynx motoneurons. The nucleus retroambiguus (NRA) is a group of the premotor neurons in the caudal brainstem. Studies in the cat have shown that one of functions mediated by the NRA is vocalization. No information is available about the relation between the NRA and vocalization related motoneurons. In the present study, the NRA projection to laryngeal motoneurons in the dorsal ambiguus complex was studied at the light and ultrastructural level. Injections with wheatgerm agglutinin horseradish peroxidase (WGA-HRP) were made into the NRA in five female rhesus monkeys to anterogradely label fibers in the nucleus ambiguus. In two of these animals, the cricothyroid muscle was injected with cholera toxin subunit b (CTb) to identify motoneurons. The results show that the NRA sends a dense projection to the compact formation of the nucleus ambiguus. The remaining parts of the dorsal ambiguus complex, including CTb-labeled motoneurons located dorsomedial to the compact formation, receive a moderate projection. T he projection is bilateral, with a contralateral predominance. At the ultrastructural level, WGA-HRP-labeled terminal profiles were found to contact CTb-labeled dendrites. The terminal profiles contained spherical vesicles and formed asymmetrical contacts. It can be concluded that the NRA in the monkey sends a strong projection to laryngeal motoneurons in the nucleus ambiguus. For the first time it has been demonstrated that projections between the NRA and larynx motoneurons are monosynaptic. FUTURE DIRECTIONS Apart from the indirect PAG-NRA-ambiguus pathway, there exists a direct PAG-ambiguus projection. Tracer injections into the PAG will be combined with tracer injections into laryngeal muscles to determine whether the direct projection from the PAG to the ambiguus complex terminates onto laryngeal motoneurons. KEY WORDS nucleus retroambiguus, larynx motoneurons, vocalization, anterograde tracing, electron microscopy FUNDING Supplemental funding to RR00167