Cochlear blood flow is considered to be closely related to cochlear function. Among several etiologic factors implicated in inner ear diseases, disturbance of local blood flow is held to be one of the most important. With this in view, various pharmaceuticals are currently being used to increase local blood flow in patients with inner ear diseases. In the control of blood flow there are three major factors; systemic blood pressure (perfusion pressure), vascular tone, and blood viscosity. Batroxobin (BX) was developed to increase local blood flow by lowering blood viscosity through defibrinogenation; it is used in the treatment of thrombosis and occasionally for the treatment of sudden deafness. In the present study, we observed the effect of BX on cochlear blood flow in guinea pigs, using a laser Doppler flowmeter, and measured the blood fibrinogen concentration after BX infusion. There was an obvious increase in cochlear blood flow during the observation period of 3 h after 10 BU/kg were infused, and a slight increase when 2 BU/kg were infused. Blood fibrinogen levels decreased dramatically by 30 min after BX infusion (10 BU/kg), and the extremely low level attained was maintained throughout the 3-h observation period. Hemorrhage from the surgically opened ear was noted in 2 animals during the experiment and rectal bleeding in one.