In the present study we examined the effect of coingestion of 50 g fat (butter) on the postprandial glucose, insulin, and gastric inhibitory polypeptide responses to 50 g carbohydrate (potato) or 50 g protein (low fat veal) in eight normal subjects. The coingestion of fat with either carbohydrate or protein resulted in greatly increased gastric inhibitory polypeptide responses, the effect being more pronounced with carbohydrate. The addition of fat to a carbohydrate meal also reduced the postprandial glucose response. This could have been due to several factors including a delayed glucose absorption, secondary to a fat-induced inhibition of gastric emptying. However, despite the lower blood glucose levels in the presence of fat the insulin response was not reduced, suggesting a potentiation of insulin secretion in the presence of fat. Thus, despite the apparent improvement in glucose tolerance when carbohydrate is ingested together with fat, the accompanying potentiation of insulin secretion could form the basis of long-term changes in insulin sensitivity which accompany alterations in dietary fat intake.