The effects of topical otic preparations (Cortisporin®, Coly-Mycin®, Aristocort®, and Bestron®) upon the permeability of the round window membrane (RWM) in chinchillas were investigated. Using K+-selective microelectrodes, the concentration of tetraethylammonium (TEA) ions was measured. Changes in the thickness of the RWM were measured using light microscopy. The RWM permeability was reduced significantly in Cortisporin-and Coly-Mycin-treated ears. Moreover, these two drugs resulted in a marked thickening of the RWM. In contrast, Aristocort or Bestron resulted in no alteration of the RWM permeabil