The effects of three different fibers on large intestinal mucosal mass per unit length, crypt morphometrics, and cytokinetics were compared by feeding fiber supplements to 40 rats for 4 wk. A control group of rats was fed a fiber-free diet and the experimental groups received the same diet, uniformly diluted by the addition (wt/wt) of 20% oat bran, 10% pectin, or 10% guar. All groups of rats exhibited equal caloric intakes and weight gains. Guar consistently produced an increase in cecal and colonic mucosal wet weight, DNA, and RNA when compared with the control group (P less than 0.05). DNA levels were increased in the cecum and proximal colon of the pectin group but remained unaltered in those rats fed oat bran. Autoradiographic measurements demonstrated that epithelial cell turnover time was longer in the distal colon of the guar and pectin groups (P less than 0.001), but shorter in the proximal colon of the oat bran group, when compared with the controls (P less than 0.01). This comparative study provides further insight into the possible mechanisms by which specific dietary fibers elicit a hyperproliferative response in the large bowel.