The ability of dietary fiber to modify the effects of a bile acid on permeability of the cecum and colon was studied. A cecal catheter, which permits administration of test materials to conscious, unrestrained rats over a period of several weeks, was designed. Rats were fed fiber-free diet or diets containing 20% bran or lignin. Permeability of the lower intestine was assessed indirectly by infusing polyethylene glycol (PEG) and measuring excretion of PEG in urine. Under these conditions there was little effect of diet on permeability of PEG. However, when sodium deoxycholate was infused with the PEG, permeability was increased in rats fed fiber-free and lignin diets. In contrast, rats fed the bran diet showed no such response to the bile acid. The interaction among type of dietary fiber, presence of bile acid and intestinal permeability may have important implications for the etiology of intestinal disease.