ABSTRACTUsing a servocontrolled micropipet pressure measuring system, the effect of intravenous and oral administration of isosorbide, a hyperosmolar agent that is an anhydrate of sorbitol, was examined in 11 fasting cats. Following intravenous administration of 2 g/kg of the agent, perilymphatic pressure has shown, on the average, a significant reduction of 136% of the initial value (3.6 ± 0.3 mm Hg). A strong rebound phenomenon, however, with a 61% rise to the initial value, was noted, and this did not come down in 3 hours. On the contrary, no rebound phenomenon was noted after oral administration, which caused a significant 60% drop in perilymphatic pressure for more than 90 minutes. Using polarographic technique, perilymphatic oxygen tension was measured and showed a maximal increase of 145% to the initial value. The time it took to reach the maximal Po2tension was equal to the time it took to come down to the minimal perilymphatic pressure. This is a strong indication that isosorbide improves perfusion and increases blood flow of the inner ear vessels.