The purpose of this investigation was to evaluate the effect of temporary ischemia created during the surgical preparation of the colon for resection on transmural migration of bacteria and passage of endotoxin through the ischemic wall of the canine colon. Eighteen dogs were used: in fourteen the colon was devascularized by ligating all marginal vessels. Aerobic and anaerobic cultures and washings for endotoxin assay were obtained from the surface of the bowel at intervals up to 6 hr after creating the ischemia. Peripheral and portal blood samples were also obtained at equal intervals for bacteriologic cultures and endotoxin assay. Bacterial transmural migration was examined in 10 dogs. In 6 of the dogs biologically marked bacteria were introduced into the colon via a rectal catheter before producing the ischemia. In 4 dogs radioactively labeled endotoxin was introduced into the colon in a similar fashion. In all the dogs, surface cultures, both routine and specific for the marked bacteria, were negative for the whole period of up to 6 hr after creation of ischemia. All portal and peripheral vein cultures were also negative. Transmural migration of endotoxin was investigated in 8 dogs, 4 of which served as controls and underwent a sham operation. In the 4 dogs in which the colon was devascularized endotoxin was discovered in peritoneal washings, and in portal and systemic blood samples, as early as 30 min after the preparation of the bowel was completed. In the control dogs endotoxin assays were negative throughout the experiment. In this model up to 6 hr of ischemia did not result in migration of bacteria through the ischemic wall. Endotoxin, however, entered the peritoneum and the blood very soon after producing the ischemia.