The purpose of this paper was to investigate the bioavailability of oxytetracycline in pathological rats and rabbits pretreated with carbon tetrachloride and mercuric chloride. The results are as follows: The blood level of oxytetracycline administered orally was mostly decreased significantly in rabbits damaged kidney and liver, and in rabbits severely damaged kidney, the blood level of oxytetracycline was not significant at 4 to 6 hours. Urinary clearance of oxytetracycline in rabbits severely damaged kidney was inhibited at 5 to 6 hours but in rabbits damaged liver. Hepatic clearance of oxytetracycline was accelerated in rabbits damaged kidney but in rabbits damaged liver. AUC of oxytetracycline orally administered in rabbits damaged liver and kidney was largely decreased. The absorption of oxytetracycline was decreased in rats damaged liver and kidney as compared with that of normal rats. Especially, absorption of oxytetracycline in rats damaged liver was more decreased than that of rats damaged kidney. The absorption of oxytetracycline was inhibited by combinated administration of carbon tetrachloride and mercuric chloride.