Diets containing a nutritionally adequate, high-maltose nutrient mixture and either 4% xanthan gum or 4% cellulose were fed ad libitum to rats. The feeding of this gum increased the combined weight of the small intestine and its contents by 110%. This effect was partially due to an enlarged intestinal cell mass and to extra dry matter in the contents but chiefly to a 400% increase in intraluminal water. Xanthan feeding enhanced greatly the persistence of sugars beyond the proximal quarter of the small intestine and increased their total recovery in the first three quarters of that organ by 150%. The xanthan-induced increase in intraluminal water in the small intestine was partially due to a slowed absorption of osmotically active substances from the gut.