The purpose of this study was to compare the effects of two highly fermentable fibers (oat bran and guar gum) with those of a less fermentable fiber (wheat bran) on the luminal environment of the large bowel. Rats were fed one of four diets containing either low fiber (2%), a highly fermented fiber (guar, 10%, or oat bran, 10%), or a medium fermented fiber (wheat bran, 10%). Short-chain fatty acids and pH showed a falling gradient along the large bowel with the low fiber, guar, and oat bran diets. However, wheat bran maintained total short-chain fatty acid levels in fresh feces at three times the levels seen with the other diets; both fecal butyrate concentrations and pH were maintained at cecal values in the distal large bowel. Thus, dietary fibers have differing effects on different regions of the luminal environment depending on their fermentability; it appears that slowly fermented fibers have a greater influence on the distal environment. Because butyrate is implicated as having an antitumor action, the variable effects of dietary fiber on tumorigenesis might be accounted for by its ability to influence distal large bowel butyrate concentrations.