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