The carbohydrate intake of seven healthy men was increased from 220-265 g/d to 620-770 g/d for 17 days, while protein and fat intake remained constant. Carbohydrate loading did not affect the preprandial plasma glucose levels after an overnight fast, but reduced the postprandial increment in plasma glucose levels after 5, 11, and 17 days of overfeeding. Preprandial plasma insulin levels were slightly increased during carbohydrate overfeeding, but no increase in the postprandial rise in insulin levels was found until 11 days after the start of carbohydrate loading. Whole-body rates of carbohydrate oxidation and of glucose conversion to fat were estimated by indirect calorimetry. Basal carbohydrate oxidation rate was increased by 95% at the end of 17 days of overfeeding, but there was no potentiation in the stimulation of the carbohydrate oxidation rate induced by a meal. There was no net fat synthesis from glucose before carbohydrate loading; carbohydrate overfeeding produced nonprotein respiratory exchange ratios greater than 1.00, suggesting net fat synthesis from glucose. Meals did not stimulate net lipogenesis from glucose, either before or after overfeeding. These results indicate that the improvement in carbohydrate tolerance associated with short-term carbohydrate loading does not appear to depend on elevated insulin levels. Increased carbohydrate oxidation and lipogenesis elevated carbohydrate disposal is more than necessary to account for the improvement in carbohydrate tolerance.