The energetic contribution from, and effects on the gastrointestinal tract of, indigestible oligosaccharides in growing rats were compared with those of sucrose (S). S and two types of oligosaccharides, fructo-oligosaccharide (Fru) and 6'-galacto-oligosaccharide (Gal) were added to a basal diet at a level of 100 g/kg. The basal diet was given either ad libitum (group B) or at a level approximately 90% of the ad libitum intakes of the Fru and Gal groups (group BR). During a 50 d feeding period, feed intake, digestibilities of nutrients, and digesta retention times using liquid (Co-EDTA) and particulate (Cr-cell-wall constituents) markers were measured. The carcass and the contents of the stomach and caecum were sampled on the last day of the experimental period. There was no significant difference in feed intake between groups other than BR. Addition of Fru and Gal to the basal diet resulted in increased crude ash digestibility and decreased crude protein and fat digestibilities. Mean retention times of digesta markers were increased by addition of Fru and Gal to the diet, and this was associated with enlargement of the caecum. Concentrations and amounts of total organic acids in the caecum were higher in groups Fru and Gal than the other groups. The amount of energy accumulated in the carcass of rats in the Gal group was significantly (P < 0.05) lower than that of rats fed on S but not Fru. Contributions to energy accumulation tended to be different between Fru and Gal; these were associated with differences in composition of caecal organic acids and of fatty acids in body fat.