An experimental model for controlled studies of an acoustic neurinoma will be described. The model employs nonhuman primates. It permits chronic experimentation. It is reversible. Essentially, the preparation is based upon the implantation of an inflatable Silastic rubber balloon in the VII and VIII cranial nerves in the internal auditory meatus of the monkey. Prior to and following such preparation, a systematic study of the contralaterally evoked stapedius reflex was performed. Afferent and efferent limbs of the reflex arc were studied. The effects of balloon inflation and deflation were examined. Variables studied included threshold across frequency, dynamic range, habituation, rate of rise, and recovery time. Normative data indicate the characteristics of the monkey stapedius reflex are comparable to man. Implantation reversibly affected all variables except habituation which was permanently affected. Changes in all variables were reversibly influenced by balloon inflation and deflation. The time course and quantitative changes observed will be described. Clinical implications and possible basic mechanisms underlying the effects of the acoustic neurinoma on auditory function will be discussed. [Work supported by NS08181 and RR00166.]