The influence of the disaccharidase inhibitor acarbose on carbohydrate tolerance was investigated in healthy subjects during substitution of fibre-free formula diets for normal food. Two separate experiments showed that acarbose was highly efficient in retarding and diminishing the postprandial rise in blood glucose and serum insulin when administered with these diets for 10 to 14 days. Acarbose decreased the area under the postprandial curves of blood glucose from 1.836 to -504 mg/dl X min in Study 1, and from 587 to -302 mg/dl X min in Study 2. The area under the serum insulin curves was reduced from 5.022 to 1.440 microU/ml X min in Study 1, and from 7.990 to 918 microU/ml X min in Study 2. In addition, acarbose greatly diminished the interindividual variation in postprandial serum insulin concentration. Its efficacy in reducing the glycaemic response to a test meal in both experiments was found to depend on the time of initiation of therapy; in contrast, the serum insulin response was only time-dependent in Study 2. Use of fibre-free diets to standardise experimental conditions proved to be a valuable tool in investigating these details.