These studies were conducted to determine to what extent dietary fibers, or related compounds such as lactulose or amylomaize starch, alter the flux of blood urea to the cecum and cecal absorption of ammonia in the rat. Cecal weight and pH values were not different among rats fed diets containing 10% lactulose, pectin or guar gum, or 25% amylomaize starch. However, the cecal wall weight was markedly higher with lactulose feeding than with the other polysaccharides, whereas volatile fatty acid concentrations were lower with lactulose. The fiber diets depressed cecal ammonia, particularly in the case of the amylomaize starch diet, whereas the lactulose diet enhanced the concentration of ammonia. Owing to cecal enlargement and enhanced blood flow, the diets containing fermentable carbohydrates promoted a higher flux of urea to the cecum and also higher ammonia absorption in spite of low concentrations of ammonia in the cecum. Lactulose led to particularly high transfer of urea and absorption of ammonia. High blood urea in rats fed a 50% casein diet led to a very high flux of urea to the cecum and, hence, to high ammonia absorption. The presence of polysaccharides amplified the flux of urea and ammonia in the cecum. This study suggests that oligosaccharides such as lactulose, although very effective for the acidification of the contents of the large intestine, may enhance cecal ammonia and its absorption. Polysaccharides such as amylomaize starch might show greater efficiency for lowering ammonia concentrations in the large intestine.