Therapeutic effect of amidotrizoate, an ionic contrast medium (76% Urografin) for sudden deafness was found serendipitously by Morimitsu in 1973.The characteristics of this therapy are the high complete cure rate of over 50% in non vertigo group, rapid recovery course, and its minimum daily dose of 1.0 ml.However, its pharmacology in the inner ear and also for sudden deafness had been obscure for last 20 years. Recently molecular biology of the epithelium of blood vessel is developing dramatically. Torihara et al made an electron-microscopic study of the blood vessels in the stria vascularis using kationized poly-L-lysine conjugated colloidal gold and made clear that the luminal surface of the endothelium is charged in negative by glycoprotein with sialic acid.On the other hand, positive charge of the marginal, intermediate and basal cells in the stria vascularis were reconfirmed by Sadanaga.Therefore, we concluded that the blood cochlear barrier is an ionic charge barrier locating between the vascular endothelium and the cells in the stria vascularis. If the negative charge is lost by unknown cause, the ionic charge barrier should be broken down and endocochlear DC potential should decrease causing sudden deafness.In this condition, the luminal surface of the endothelium will be charged in positive by remaining function of the strial cells and the negative ionic contrast medium (Urografin) will bond ionic on the luminal surface making a pseudo-barrier which should be effective for the recovery of the original ionic charge barrier. In conclusion, sudden deafness should be a disease due to break down of the blood cochlear barrier in the stria vascularis and the pharmacology of the contrast medium should be an ionic, or molecular bonding on the luminal surface of the endothelium.