It is known that the gut may serve as a reservoir for various microorganisms, which under specific circumstances may intrude into the systemic circulation, causing systemic infections. The aim of the present study was to estimate the "critical time" of disruption of the small-intestine mucosal barrier in conditions of experimentally induced intestinal occlusion, based on the histopathological alterations observed under light and electron microscopy. Forty rabbits underwent small-intestine obstruction through ligation with a nonabsorbable suture. Blood cultures from portal vein and inferior vena cava, as well as cultures from the peritoneal fluid, a hepatic fragment, and a mesenteric lymph node, were obtained before the ligation (0 h). The same cultures were repeated at 4 and 8 h (group A, 20 rabbits) and at 6 and 12 h after the ligation (group B, 20 rabbits). Small-intestine specimens proximal to the occlusion were taken for examination under the optic and electronic microscope in the same time intervals. Five of 20 rabbits of group A died within 4 h and 6 of 20 rabbits of group B died within 6 h after the operation. All anaerobic cultures were negative. All aerobic cultures that became positive developed Escherichia coli colonies. Intestinal epithelium of dead animals was transformed to cuboid with destruction of goblet cells and alteration in secretion of acid polysaccharides. The mucosal appearance of all rabbits that survived 12 hours after ligation was the same. The disruption of the mucosal barrier begins 4 h after complete intestinal occlusion. At 12 h after complete intestinal occlusion, the disruption is total with different degrees of severity.