Excretion of creatinine into gastrointestinal tract was studied using renal-artery-ligated rats as a model of patients suffering from renal failure. In the rats of first group, both arterial branches of kidney and bile duct were ligated, and a saline solution was sent through the intestinal lumen to perform the intestinal dialysis. In the second group, after ligation of both renal arteries, a fine catheter was sutured with bile duct to collect the bile. After the intravenous administration of creatinine (138 mg or 34 mg/kg) through the femoral vein, the animals were kept for 1 hr until the creatinine level in body-fluid approach to steady state. Under these steady state conditions, about 170 μg or 70 μg of creatinine was permeated into gastrointestinal tract every 1 hr from upper and lower region of jejunal mucosa after the administration of 138 or 34 mg/kg of creatinine, respectively. The creatinine concentration in bile was similar to that in blood and about 200-270 μg or 90-110 μg of creatinine was excreted into bile every 1 hr after the injection of 138 or 34 mg/kg of creatinine, respectively. Creatinine excreted from the intestinal mucosa and bile was collected together by perfusing the saline solution for 3 hr. About 3.5% of creatinine was excreted. These results suggested that, at least, 20-30% of creatinine in body fluid may be excreted daily into gastrointestinal tract through the intestinal mucosa and from bile in the renal shutdown rats.