PURPOSE:Barrier properties of an isolated colon loop and the remnant colon in continuity with the gastrointestinal tract after colostomy were studied in the rat.METHODS:The in vivo absorption after colonic loop administration of the marker fluorescein sodium was measured as the urinary recovery. The in vitro permeability was measured in Ussing diffusion chambers as the transmucosal passage of [14C]mannitol and of human serum albumin in the isolated and the nonexcluded colonic segments and was compared with the corresponding colonic regions from sham‐operated rats at 1 to 14 days after operation.RESULTS:Body weight gain of the rats decreased and diarrhea appeared from day 2 after colostomy. Histologic examination showed mucosal atrophy with decreased villus height in the isolated colonic loop and an increased villus height in the nonexcluded colon segment. Absorption of fluorescein sodium in the isolated loop was increased at 8 and 14 days. Moreover, permeability in the isolated loop was increased for both mannitol and human serum albumin from four days after colostomy compared with the corresponding colonic segments after the sham operation, whereas a decrease in the passage of mannitol was noted in the nonexcluded colon.CONCLUSIONS:Experimentally performed colostomy diversion in the rat induced alterations of the barrier function in both the isolated colonic loop and the nonexcluded colon in continuity with the fecal stream.