Glycerol, an osmotic diuretic, has been used for the diagnosis and treatment of endolymphatic hydrops. Hearing improvements in hydropic ears are attributed to its dehydrating effect. In addition to this effect, glycerol also increases cochlear blood flow. Urografin, another hyperosmotic agent used for vasography, is similarly known to increase local blood flow. The present study compared these two hyperosmotic agents, glycerol and Urografin, in their effects on cochlear blood flow and serum osmolarity. Laser Doppler flowmetry on the lateral wall of the cochlea revealed that the increase in cochlear blood flow with a 30-min infusion (0.025 ml/min) of 76% Urografin continued for a longer time than with a 30-min infusion (0.025 ml/min) of 50% (v/v) glycerol. The significant increases appeared at 20 and 30 min after the infusion with the former; 10, 20, 30, 40, 50 and 60 min after the infusion with the latter. Intravenous infusion of these agents also caused elevation in serum osmolarity. This elevation was appreciably greater with Urografin infusion (maximal increase: about 30 mOsm on average) than with glycerol infusion (maximal increase: about 6 mOsm on average), and the former elevation appeared to be longer lasting than the latter. These differences were ascribed to differences between glycerol and Urografin with respect to the creation of an osmotic gradient across the capillary walls of cochlear blood vessels. Since glycerol penetrates the interstitial space and moves into inner ear fluids, the gradient may decline faster. It would be assumed that a higher concentration of the hyperosmotic agent in the capillary blood causes more vasodilatation and lowering of blood viscosity. Alternatively, direct action of these agents on the vascular wall may affect some biological processes, leading to vasodilatation in different degrees and durations with different agents. Hearing improvement with glycerol administration in hydropic ears was also discussed from the perspective of cochlear blood flow.