Serial sections of the medulla of Lepus cuniculus were subjected to Koelle and Friedenwald''s acetylcholinesterase test; a 2nd group of serial sections were Nissl-stained and a 3rd group were studied by Cajal''s reduced silver method. Retrograde cell changes occurred following intracranial transection of the 8th nerve or electrocoagulation of the cochlear as posterior labyrinth. Changes in acetylcholinesterase activity in the membranous labyrinth occurred by transection of the vestibular nerve before it emerges from the medulla. These studies confirm the findings of previous experiments on the guinea-pig and demonstrate the existence of 5 bundles of efferent fibers connecting the medulla oblongata to the inner ear. A bundle of uncrossed efferent fibers exist which arise from cells in the reticular formation of the pons and medulla situated to the side of the median raphe and travelling to the cochlear, ampullary and macular receptors (direct reticulo-cocnlear and vestibular bundle). Two bundles of uncrossed efferent fibers to the receptors of the posterior labyrinth were found. The 1st of these 2 bundles, termed "direct ventral efferent vestibular bundle", arises from a small nucleus 1st observed in the guinea-pig and named "interposed vestibular nucleus". This nucleus is situated ventrally to the lateral vestibular nucleus and dorsally to the inferior vestibular nucleus. The 2nd of these 2 bundles, named "direct dorsal efferent vestibular bundle" arises from the antero-caudal part of the lateral vestibular nucleus. Distinct in origin and course, these 5 bundles form the "efferent cochlear and vestibular system". On the lateral border of the descending root of the 5th nerve they combine to form a single bundle, which emerges from the medulla oblongata together with the vestibular branch of the 8th nerve, and radiate to the cochlear and vestibular receptors.