Dietary Fibre is metabolised in the human colon. The inaccessibility of the human colon necessitates the use of indirect methods to study the effects of fibre upon metabolism and stool weight. The time duration required for dietary trials, to insure adaptation to the new regime is completed, is not known. This thesis describes an animal model which could be of relevance to human nutrition for the evaluation of dietary fibres and their action upon stool bulking and caecal metabolism. At present there is no simple method of predicting quantitatively the effect of dietary fibre upon metabolic and physiological functions. Adult male Albino Wistar rats, were fed for periods of 4-, 8- and 12 weeks with low fibre containing diets, either of plant origin, animal origin or an elemental diet, unsupplemented or supplemented with 100 g/kg of gum arabic (readily fermented polysaccharide) or coarse Canadian Red Spring Wheat Bran (non-fermented fibre). The effects on live-weight, liver weight, dry stool weight, and caecal content weight, wet caecal sac weight, faecal- and caecal short chain fatty acids, average bile acids, expired hydrogen and methane were recorded. Bacterial mass was measured indirectly using 2,6-Diaminopimelic acid. Wheat bran alone significantly increased dry stool weight, irrespective of the basal diet. As with all the parameters measured, the absolute values, and magnitude of the changes, were related to basal diet. Gum arabic had no influence on stool weight. The addition of gum arabic to a diet increased the caecal sac wet weight and dry caecal content weight. Total, and concentrations of caecal and faecal bacterial mass and short chain fatty acids increased with gum arabic. The addition of gum arabic increased the major anion, acetate, in caecal and faecal material. The presence of wheat bran increased the molar proportion of caecal butyrate. Isomeric forms appeared with the animal origin and elemental diets. Methane production in rats on gum arabic diminished with time. Wheat bran abolished methane production. Gum arabic increased total caecal bile acids, in particular, the muricholic acids. The effect of bran was less clear cut and varied with the basal diet given. The various interactions between basal diets, supplements and time were recorded. The basal diets influenced the way in which the two fibre supplements behaved. The differences were both quantitative and temporal. Basal diet, duration of feeding and supplement have noticeable effects upon all the measurements made. For the initial assessment of dietary fibre, a 4-week stool collection maybe adequate. For metabolic analysis a longer period is required. Gum arabic was found to alter all metabolic events, but not stool weight. Wheat bran influenced stool weight, indicative of the caecal degradation of these fibres. These results are in agreement with previous work and confirm that the rat is a suitable animal model for the routine evaluation of dietary fibres.