The mechanism for the ameliorating effect of dietary fiber on rat intestines was investigated by using Tween 20 and amaranth as toxic chemicals. Ingestion of the diet containing 10% Tween 20 or 5% amaranth induced marked decreases in the body weight gain, food consumption, and segmental sucrase activity in the small intestine. It also induced severe diarrhea in the rats. These adverse effects of the chemicals were prevented by the concurrent ingestion of Gobo dietary fiber (GDF). We found that the amount of concanavalin A-binding glycoprotein (CBGP) in the feces of rats which had ingested the chemicals with GDF was much larger than in that of the rats fed only on the basal diet. In addition, the toxic chemicals or GDF alone in the diet resulted in a definite change in the electrophoretic profile of fecal CBGP, while the concurrent ingestion of GDF and the chemicals produced no change in the electrophoretic profile. These results suggest that renewal of the epithelium caused by the addition of dietary fiber overcome the adverse effects from solubilization and exfoliation of the brush border membrane by those chemicals.