For 40 years, the amino acid acetyl-dl-leucine (or isoleucine/Tanganil®) has been used in clinical practice to reduce the imbalance and autonomic signs associated with acute vertigo crises. In animal models, acetyl-dl-leucine was shown to accelerate vestibular compensation following unilateral labyrinthectomy, while having only minor effects on normal vestibular function. However, the underlying mechanisms are unknown. In this study, the effect of acetyl-dl-leucine on the activity of central vestibular neurons of the medial vestibular nucleus (MVN) and/or the overall activity of vestibular-related networks was electrophysiologically measured in brainstem slices and in the isolated, in vitro whole brain (IWB) of guinea-pig. Only moderate effects were obtained in normal animals, where both excitatory and inhibitory actions of acetyl-dl-leucine were obtained. However, intracellular recordings from MVN neurons revealed that the nature of the response depended on the resting membrane potential. The neurons excited by acetyl-dl-leucine were significantly hyperpolarized compared to nonsensitive cells, whereas the neurons inhibited by this compound tended to display higher than normal membrane potentials. In accordance with these data, acetyl-dl-leucine reduced the prominent asymmetry characterizing the vestibular-related networks of IWBs taken from previously labyrinthectomized animals, by decreasing the activity of the abnormally depolarized neurons on the hyperactive side. Altogether, our results suggest that acetyl-dl-leucine might act mainly on abnormally hyperpolarized and/or depolarized MVN neurons, by bringing back their membrane potential towards a mean value of −65 to −60 mV. Since in animal models, acute vestibular disorders are associated with asymmetrical spontaneous activities of MVN neurons, this study suggests how acetyl-dl-leucine may reduce acute, vestibular-related imbalances in humans.