The absorption, distribution, metabolism and excretion of E3810 were studied in rats after administration of [benzimidazole-2-14C] E3810 (abbreviated as 14C-E3810) or [alpha methylene-14C] E3810. After a single intraduodenal dose of 14C-E3810 (20 mg/kg) to rats, the blood level of radioactivity reached maximum at 15 minutes (3.86μg eq./ml of blood) and then was eliminated extremely slowly. In rats, the radioactivities in blood cells and thyroid tissue remained for a long time following administration of 14C-E3810. On the other hand, radioactivities in these tissues after dosing of [alpha methylene-14C] E3810 were eliminated faster than after dosing of 14C-E3810. Therefore, the long half life time in blood cells and thyroid tissues after dosing of 14C-E3810 should be attributed to the moiety of benzimidazole ring labeled with 14C. After addition of 14C-E3810 to blood in vitro, the incorporation of 14C to blood cells in rats was higher than these in dogs and in humans. After intraduodenal administration of 14C-E3810, the radioactivity rapidly distributed to the tissues except for the thyroid and the distribution of radioactivity into the brain was very low. The radioactivity in the tissues were rapidly eliminated, except the blood cells and the thyroid. Distribution of radioactivity to gastric mucosa, the target tissue, after intravenous dose was shown clearly by the whole body autoradiography. The main metabolites in plasma were UM-1, and UM-2, with small amounts of unchanged compound, M-1, and M-2 at 15 minutes after dosing of 14C-E3810. In the hepatic portal vein, over 70% of the radioactivity was recovered as unchanged compound after intraduodenal dose in situ experiment, therefore it was suggested that E3810 is mainly metabolized in liver. The urinary and fecal excretions of radioactivity were 39.75% and 56.32% of dose, respectively for 8 days after intraduodenal administration of 14C-E3810 to rats. The biliary excretion of radioactivity was 60.86% of dose 24 hours after intraduodenal administration in bile duct-cannulated rats.