The isolated small bowel from duodenojejunal junction to ileocecal junction with intact SMA and SMV was perfused with Krebs-Ringer solution intraluminally in rat. After all the branches from aorta and portal vein other than SMA and SMV were ligated, aorta and portal vein were cannulated and perfused with Krebs-Dextran solution. The isolated perfused bowel was transferred immediately to the chamber equipped with constant temperature and humidity. For the purpose of studying the direct toxicity to the bowel tissue, B4 endotoxin (Difco) was added to intravascular perfusate. Significant physiological and chemical changes listed below were observed as compared with those of the control group which was perfused with endotoxin free perfusate. Active transport of D-glucose was decreased. Net entry of water into the gut lumen was decreased. pH in intravascular fluid was significantly decreased. Lactic acid level in intravascular fluid was significantly high, which was well correlated with pH change. O2 consumption change was not marked, but CO2 accumulation in intravascular perfusate was significant. These were the results of the toxic effects of endotoxin on the rat bowel.