Effects of dose administration（perfused at 100,200,400 mg/kg）of sulfadimethoxine sodium on the serum biochemical parameters including aspartate aminotransferase (AST),alanine aminotransferase (ALT),alkalinephoshatase (ALP),lactate dehydrogenase (LDH),creatinine (CRE) and urea (UREA),and the structure of liver and kidney of channel catfish (Ictalurus punctatus) were studied.No differences were found in the activities of AST,ALT,ALP,LDH and the content of UREA between the control group and the treated groups after consecutively perfused for 5 days.But the content of CRE increased significantly at the dose of 400 mg/kg.After consecutively perfused for 10 days,the activites of AST,ALT,ALP and LDH,and the contents of CRE and UREA changed obviously among the groups.From the tissue slices of liver and kidney in the 10th day,the livers and kidneys were damaged by SMS,inducing the cell lysis of liver and epithelial cell of tubular necrosed,and the higher the concentration,the greater the damage.Therefore,after long-term use of 100 mg/kg SMS,there was less toxic and side effects,and it was safe in 5 days.The dose of 200 mg/kg SMS was safe in a short time,otherwise the structure of liver and kidney would be damaged.The high dose of SMS had toxic damage on the structure of kidney in a short time.