An increase of perilymphatic fluid pressure was found to be an important factor in the aetiology of round window membrane rupture. The critical pressure causing membrane rupture, as determined in anaesthetized cats, was in the range of 10-30 mm Hg (mean, 23.4 +/- 17.1). This value was compared to the pressure increases caused by thoracic and abdominal compression, cervical strangulation, forced Trendelenburg position, coughing, and sneezing.