In an attempt to determine why sodium saccharin feeding causes caecal enlargement (Anderson & Kirkland, Fd Cosmet. Toxicol. 1980, 18, 353) and increased stool hydration (Anderson, Fd Cosmet. Toxicol. 1979, 17, 195), stools from rats fed diets containing 0, 1, 3, 5 or 7.5% sodium saccharin were analysed. Saccharin ingestion resulted in a small increase in stool ash but no change in lipid or non-saccharin nitrogen concentrations (mg/g dry stool). Saccharin treatment also resulted in a dose-dependent increase in the stool content of carbohydrate soluble in 1 N-NaOH. The results have led to the hypothesis that saccharin feeding results in a dose-dependent increase in the stools in the content of hygroscopic polysaccharides which may be derived from the diet or synthesized by an intestinal microorganism(s). The polysaccharide in conjunction with the high stool saccharin content causes caecal enlargement and increased stool hydration. A possible relationship between this effect of saccharin and urinary-tract toxicity is suggested.