1. The present study was performed in order to assess the influence of a high-carbohydrate diet on carbohydrate and lipid metabolism in humans. 2. Short-term (3 weeks) and long-term (6 weeks) effects of a high-carbohydrate diet (HCD, 85% carbohydrate) were compared with the effects of 2 weeks on a diet of normal carbohydrate content (ND, 45% carbohydrate). The study was conducted on 4 normal and 11 obese subjects (with normal or impaired glucose tolerance) on a metabolic ward. Fasting blood glucose, insulin and triglyceride levels were measured twice a week. Tolerance to carbohydrates was evaluated by measuring blood glucose and insulin levels after oral and intravenous glucose loads and liquid meals at intervals throughout the 8-week period. 3. Long-term HCD improved the tolerance to oral glucose, i.e., it decreased serum glucose and insulin responses to an oral glucose load. 4. HCD had no effects on fasting levels of glucose or insulin or on the tolerance to an intravenous glucose load. 5. A high-carbohydrate liquid meal induced greater post-prandial glucose and insulin levels than a normal liquid meal. 6. In a second group of 6 normal and 6 obese subjects, efficacy of the liquid meal tests was evaluated. A high-carbohydrate liquid meal induced greater glucose and insulin responses than a normal liquid meal during both dietary periods. 7. Fasting triglyceride levels increased and remained high throughout the HCD period.