1. A comparison was made of the effect of a fibre-free diet and diets containing non-digestible polysaccharides on rat caecal and colonic physiology and microflora. 2. All polysaccharide-containing diets led to enlargement of the caecum and colon, associated with increased weight of contents, and of tissue. Carboxymethylcellulose (CMC) had the most marked effect and animals given this also had watery faeces. 3. The density of bacteria in the caecum and colon varied significantly with diet and the proportion of aerobic bacteria in the flora was increased by the CMC diet. 4. In vitro, CMC and hydroxypropylmethylcellulose were poorly fermented. 5. There was a high correlation (caecum r 0.93; colon r 0.94) between tissue weight and wet weight of organ contents but no correlation with bacterial density, number of bacteria per organ, moisture content or short-chain fatty acid content. 6. It is concluded that caecal and colonic enlargement is due to tissue hypertrophy in response to increased bulk of contents, irrespective of the nature of that bulk which varies with diet; it is unlikely that short-chain fatty acids or other microbial metabolites are the stimulus for the trophic response seen when non-digestible dietary polysaccharides are fed to rats.