The fate of cereal fructans and inulin in the gastrointestinal tract was investigated by using a rat model. Male Wistar rats received diets containing 5.0% of a preparation of cereal fructans or 4.7 or 9.4% inulin. Each diet was tested with or without Nebacitin, an antibiotic drug given to suppress the intestinal microflora. The availability of fructans in the upper gastrointestinal tract was calculated from the recovery in feces in Nebacitin-treated rats. Animals receiving cereal fructans developed diarrhea, which made it impossible to measure the bioavailability of this substrate. With inulin, the extent of digestion and absorption appeared to be 18-26%. This might, at least to some degree, be explained by hydrolysis by gastric juice in the stomach. Thus, in an in vitro assay, mild acid hydrolysis (0.05 M HCl, 37 degrees C, 120 min) converted about 8% of inulin to free fructose. The apparent digestibility of the fructans was calculated from its recovery in feces from untreated rats. Only minute amounts of fructans could be detected, hence, cereal fructans as well as inulin reaching the hindgut appeared to be almost completely fermentable.