THE EXISTENCE and course of an efferent component in the vestibular nerve branches has been demonstrated by experimental neuroanatomical techniques.3,8,9 Acetylcholinesterase (AChE) activity of this efferent system has been localized in the vestibular sensory epithelium of several species.2,5,6,10,11 This histochemical study illustrates the distribution of AChE activity in the efferent nerve fibers within the vestibular nerve and its branches. The advantages of the employed technique are discussed elsewhere.1,4 Materials and Methods Twenty albino guinea pigs were used. The animals were decapitated under ether anesthesia. The petrous bones were removed, trimmed, and decalcified in four days with cold 10% disodium edetate (EDTA) in a 0.1 M phosphate buffer solution at pH 7.4.1 Afterwards, the specimens were frozen on solidified CO2 (Dry Ice) and sectioned at 20μ in a cryostat at −18 C. The sections were mounted on clear cover slips, and briefly dried. Then they were incubated