The ultrastructural abnormalities produced in the stria vascularis by intravenous furosemide (80 mg/kg) were investigated in 14 guinea pigs. The changes consisted of marginal cell swelling, shrinkage of the intermediate cells and enlargement of the intercellular spaces, as described in other intoxications. The cytological derangements (including characteristic dilatation of the Golgi membranes) differed in detail from those arising after a comparable dose of ethacrynic acid. The morphological alterations were already present at 2 min, were maximal at 10 min, recovered only slowly at first and had not disappeared entirely at 180 min. For comparison, the fall in the endocochlear potential had a latent period of 20 s and was greatest at 2.3 min; its recovery was rapid initially but also incomplete at 180 min. Thus, no gross discrepancy in the time courses occurred, even if the correlation was imperfect. That reported previously must be due, therefore, to the much longer delays found following intraperitoneal administration.