The effects of feeding soluble dietary fibre (inulin, I) separately or in combination with insoluble fibre (cellulose, CEL) on caecal fermentation and biochemical blood parameters were studied in rats. Four diets: CEL-rich (10%) and CEL-free, each supplemented with 5% of sucrose or I (CR, CF, CRI and CFI diets, respectively) were fed over 4 weeks, each to 10 male growing Wistar rats. Supplementation with I caused the increase of the mass of caecal tissue and digesta (only on CF diet, interaction at P<0.01 and P<0.05, respectively), modification of short-chain fatty acids profile, decrease of caecal pH and ammonia concentration, increase of Bifidobacteria and depression of E. coli number, and decrease of β-glucuronidase activity. Feeding CEL reduced ammonia and total SCFA concentration and activity of all enzymes, particularly β-galactosidase, did not affect microbial population but slightly decreased number of anaerobic saccharolytic and proteolytic bacteria spores. Interactive effects of I and CEL were found for the concentration of branched-chain fatty acids which was more depressed by I on CF than on CR diet. Dietary treatments did not affect blood parameters.