In a four-week experiment on 60 7-day-old BUT-9 male turkeys the effects of dietary fructooligosaccharides (pure nystose and a fructooligosaccharide mixture) supplemented at 1 and 2%, were studied on ileal and caecal metabolism. The control carbohydrate was cellulose, added also at 1 or 2%. Each dietary treatment consists of 10 birds kept individually. The average degree of polymerisation of the nystose and oligofructose preparation amounted to 2.9 and 4.1, respectively. The addition of nystose significantly decreased the pH value and viscosity in the ileal contents compared with the cellulose treatment. On the other hand, the oligofructose preparation increased the activity of sucrase and lactase in the ileal mucosal by 30-60% and 33-47%, respectively. Both fructan preparations similarly acidified the caecal and colonic digesta (by 0.2-0.4 pH units) as well as diminished the activity of bacterial harmful beta-glucuronidase (by 24-40%), but only nystose caused an enlargement of the caeca and effectively reduced caecal ammonia concentration, especially at a higher dose. Oligofructose supplementation at 2% caused a 3.5-fold increase of bacterial activity of alph- and beta-galactosidase, while 2% nystose resulted in 1.7 and 3 times higher alpha- and beta-glucosidases activities, respectively. Compared to oligofructose, dietary nystose increased propionic and decreased butyric fermentation in caeca. Nystose and oligofructose preparations added at 2% reduced the triacylglycerol concentration in the serum in comparison to the addition of 2% cellulose by 46 and 25%, respectively. Beside the fact that dietary levels of supplementation were of great importance, the results indicated that even small difference in the length of carbohydrate chain may cause different physiological responses.