The present study was undertaken to investigate depressor action of tannic acid and the mechanism underlies it in the rabbit. The changes in arterial blood pressure were studied after intravenous administration of tannic acid in normal rabbits and the animals pretreated with atropine, propranolol, dibenamine, and hexamethonium. The results obtained were as follows; 1) Following administration of 1.5 mg/kg, 3.0 mg/kg, and 5.0 mg/kg of tannic acid into rabbits the maximum depressor responses observed were 12.0±0.9 mmHg, 23.4±1.0 mmHg, and 34.0±1.8 mmHg respectively and generally depressor responses increased in proportion to dosage of tannic acid. 2) Since there were no changes in depressor responses to tannic acid in animals pretreated. separately with atropine, propranolol, dibenamine, and hexamethonium, the depressor responses appeared to be resulting from direct vasodilator action of tannic acid on the vascular smooth muscle. Comparing tannic acid and acorn extract in their mechanisms of depressor responses, it is strongly indicated that in acorn there might exist another depressor substance. 3) After administration of large doses of tannic acid, in addition to respiratory changes, the mean arterial blood pressure decreased markedly and was never recovered throughout the experiment. comma Therefore it is also suggested that the lethal action of tannic acid resides in a drastic decline of arterial blood pressure and in respiratory changes as well.