We report on a case of reversible pontine deafness caused by multiple sclerotic lesions in the pons, and describe six experiments in humans, in whom we recorded transient otoacoustic emissions and distortion products. Our findings indicate an inhibitory effect of efferent innervation on the motility of outer hair cells in humans. Otoacoustic emissions increased in patients with unilateral deafness after the central part of the efferent pathway was destroyed (pontine deafness). Otoacoustic emissions also increased in patients with myasthenia gravis after the administration of an acetylcholinesterase inhibitor. General myotonia (Batter-Curschmann-Steinert Syndrome) resulted in mild sensorineural hearing loss, and in the absence of contralateral inhibition of otoacoustic emissions. Otoacoustic emissions decreased gradually under general anesthesia with muscle relaxation. Contralateral acoustic stimulation was seen during anaesthesia with muscle relaxation.