Acute and chronic experimental ulcerative colitis models were produced in mice by providing them with drinking water containing synthetic dextran sulfate sodium. Mice that developed acute colitis showed signs of diarrhea, gross rectal bleeding, and weight loss within 6-10 days after ingesting 3%-10% dextran sulfate sodium. On postmortem examination, multiple erosions and inflammatory changes including crypt abscesses were found on the left side of the large intestine. Mice that developed chronic colitis showed signs of erosions, prominent regenerations of the colonic mucosa including dysplasia, shortening of the large intestine, and frequent formation of lymphoid follicles after 5 administration cycles, where each cycle was composed of 7 days' consumption of drinking water containing 5% dextran sulfate sodium followed by 10 days' consumption of distilled water. The population of intestinal microflora, Bacteroides distasonis and Clostridium spp., increased significantly in mice with acute and chronic ulcerative colitis. Further, morphological studies suggest that the administered dextran sulfate sodium was partially phagocytized by macrophages in the colonic mucosa.