The relationship between dietary fiber and intestinal circular muscle cell size was investigated in rats by feeding defined diets supplemented with four different sources of fiber. In the first study, a 20% wheat bran supplement was fed to 10 rats for nine weeks. This resulted in larger muscle cell size, with a 22.5% increase in the proximal (P less than 0.02) and 77.9% increase in the distal colon (P less than 0.01) when compared with a control group of 10 rats fed a fiber-free diet. In the second study, which lasted four weeks, a control group of 10 rats was fed a fiber-free diet, while similar sized experimental groups were fed the same basal diet plus either 20% oat bran, 10% pectin, or 10% guar. Muscle cell size was decreased by 20.6% in the proximal jejunum of the oat bran- and pectin-fed groups (P less than 0.05) and by 43% in the proximal colon of the oat bran-fed group, when compared with the controls (P less than 0.05). These results show that the effects of high fiber diets on intestinal muscle cell size depend on the type of fiber consumed.