The effect of guar on the plasma glycine response to a glycine drink was measured in six normal subjects. Guar reduced the increment in plasma glycine over the first 60 min and also reduced the total integrated plasma glycine response. The effect of guar on the peripheral blood glucose, insulin, C-peptide and glucagon responses following jejunal amino acid or glucose perfusion was measured in nine normal subjects. Guar reduced the blood glucose, insulin and C-peptide responses to the glucose perfusion and reduced the insulin and C-peptide responses to the amino acid perfusion. The addition of guar did not alter C-peptide:insulin ratios following either glucose or amino acid administration. It is concluded that guar reduces the rate of amino acid absorption and of amino acid-stimulated insulin release. Modulation of intestinal mechanisms is sufficient to mediate the latter effect. The reduction in peripheral blood insulin levels caused by guar is not associated with a change in hepatic insulin extraction.