We studied the effects of a natural sodium alginate (isolated from Laminaria angustata Kjellman var. longissima Miyabe, Phaeophyceae) (average molecular weight: 2700 kDa; AG-270) and three water-soluble low-molecular weight sodium alginates (average molecular weights, 10, 50 and 100 kDa; AG-1, AG 5, and AG-10, respectively) on cholesterol excretion and glucose tolerance in rats. AG-270, AG-5 and AG-10 enhanced cholesterol excretion into faeces. AG-270 and AG-10 inhibited blood glucose and insulin levels from rising 30 min after glucose administration. AG-5 inhibited the blood glucose level from rising 30 and 60 min after glucose administration, without affecting blood insulin levels. AG-1 had no effect on cholesterol excretion or glucose tolerance. These findings suggest that the effects of the natural sodium alginate and AG-5 and AG-10 on cholesterol excretion and glucose tolerance may be due to the inhibition of cholesterol and glucose absorption from the small intestine by the gelling of the free alginic acid converted in the stomach. These experimental results indicate that the low-molecular weight sodium alginates, AG-5 and AG-10, should be useful as dietary fibers for the prevention of obesity, hypercholesterolemia, and diabetes.