Publisher Summary Deiters' nucleus and the medial vestibular nucleus influence cervical and thoracic motoneurons by means of the lateral and medial vestibulospinal tracts. The former consists of excitatory, the latter, at least in part, of inhibitory fibers. The two tracts act as labyrinthine relays to the cells innervating the neck and back muscles, but the activity of the cells of origin is also modulated by other inputs. Excitatory and inhibitory connections between the lateral and medial vestibulospinal tracts (VST and MVST) fibers on the one hand, and neck motoneurons on the other, are monosynaptic, permitting direct and precise vestibular control of the neck musculature. By means of pathways, at least disynaptic, excitatory VST fibers originating in Deiters' nucleus facilitate and inhibit extensor and flexor forelimb motoneurons respectively. In its actions on these neurons, the VST does not act primarily as a labyrinthine relay. No MVST action has been observed in forelimb neurons. This pattern of connections here resembles that observed in hindlimb motoneurons, with the exception that the VST makes some monosynaptic connections with the latter.