The effects of increasing amounts of differing single-fiber sources in chemically-defined and nutritionally-equivalent diets on morphometric measurements of the rat small and large intestinal mucosa were examined. Male Wistar rats received a fiber-free diet, or diets containing 4.5% or 9.0% cellulose, pectin, or hemicellulose. Cellulose and pectin increased jejunal (but not ileal) villus and crypt heights, whereas hemicellulose increased ileal (but not jejunal) mucosal height. Moreover, cellulose and pectin (but not hemicellulose) increased crypt heights in cecum and proximal (but not distal) colon. Parallel increases in the mitotic index were seen with cellulose and pectin diets, but there was a decrease with hemicellulose, suggesting different mechanisms for altered patterns of cell renewal induced by orally-administered single-fiber sources. These changes indicate that differing single-fiber sources exert differential structural effects along the length of the small and large intestine in the rat.