The masseteric reflex elicited by electrical stimulation of the mesencephalic trigeminal nucleus (MTN) was influenced by electrical stimulation of the central stump of the vagal nerve and of the reticular formation in the duck; the vagal nerve exerted predominantly inhibition while the reticular formation could inhibit or enhance the jaw reflex movements: increase was the most common effect. Unitary discharge of the MTN was recorded in curarized ducks by means of tungsten microelectrodes. Units were identified as being typically influenced either by movement of the jaw or by stretch of the "masticatory" muscles; the analysis of their electrophysiological properties permitted to qualify them as first-order neurons of the "masticatory" muscle spindles. Units of the MTN were modulated by single-shock or repetitive electrical stimulations of the central stump of the vagal nerve and of the brain stem reticular formation. The predominant effect of the vagal nerve was inhibition, while the reticular substance elicited both facilitatory and inhibitory effects.