The effect of inulin on diet intake, excreta output, microbial ecosystem and concentrations of volatile fatty acids in rat's caecum was investigated on twenty one, 8-week old male Wistar rats. The animals were divided randomly into three experimental groups of 7 rats each, which were kept in individual cages. Three experimental isoprotein and isoenergetic diets were prepared providing different source and amount of fermentable carbohydrate. The control diet contained 5% of potato starch, while the IN-5 and the IN-10 diet contained 5 and 10% of inulin, respectively. Rats were fed these diets and provided distilled water ad libitum for 10 weeks. Dietary intake was monitored daily, weight gain was recorded twice a week. Between 30 and 41 day of experiment 10-day balance study was performed to determine diet intake and excreta output. Transit time was determined with the help of chlorophiline (0.5% diet weight) as a colour marker. At the end of experiment, after 12-h starvation, rats were sacrificed by thiopental injection and dissected to obtain samples of caecal digesta. Inclusion of 5 and 10% of inulin in the rat's diet did not affect significantly feed intake, excreta output, its water content and transit time. However, in rats fed high-inulin diet a tendency to a higher water content of excreta and decrease of transit time was observed. Concentration of the volatile fatty acids in caecum, specially the butyrate, was significantly higher in rats fed inulin supplemented diets, however without changing their mutual proportions. Inulin inclusion in the diet, led to changes in caecal microflora populations, total counts of the coli form and anaerobic bacteria dropped. Comparing to control group the 10%) inclusion of inulin led lowered the count of total coli form and anaerobic bacteria.