Effects of selected undigestible gel-forming complex carbohydrates, including locust bean gum, guar gum, and pectin, on glucose tolerance and food motility were studied in rats. Addition of 2.5% of guar gum or locust bean gum to an oral glucose tolerance test solution significantly altered the postprandial serum glucose response. Although all three complex carbohydrates tested did not reduce the initial rise in serum glucose, locust bean gum and guar gum significantly reduced its subsequent rebound hypoglycemia. Further tests with locust bean gum showed that these effects were dependent on the concentration of the gum added to the test solution or diet. Addition of locust bean gum to test diets reduced the rate of gastric emptying and thus slowed down the passage of food from the stomach into the upper small intestine. The study suggests that addition of locust bean gum to the diet can flatten the postprandial serum glucose curve by slowing the rate of food passage from the stomach into the small intestine. It is probable that locust bean gum and other similar materials may be useful as an adjunct dietary treatment of diabetes mellitus in humans.