Fourteen patients with non-insulin-dependent diabetes (NIDDM) attended the study centre on 4 mornings separated by at least 3 days, to receive in random order 75 g carbohydrate breakfast meals of control or guar breads with jam and butter. Guar gum flours of low, medium, and high molecular weight (MW) were incorporated into wheat bread rolls to provide 7.6 g guar per meal. Venous blood samples were taken via an indwelling cannula in a forearm vein at fasting and at eight postprandial times and then analysed for blood glucose, plasma insulin, C-peptide, and gastric inhibitory polypeptide (GIP). Guar gum bread significantly reduced the postprandial rise in blood glucose, plasma insulin, and, except for bread containing low MW guar gum, plasma GIP levels compared to the control. Thus, the partial depolymerization of guar gum does not diminish its physiological activity. No reductions in postprandial plasma C-peptide levels were seen after any of the guar bread meals. This suggests that guar gum attenuates the insulin concentration in peripheral venous blood in patients with NIDDM by increasing the hepatic extraction of insulin.