A 50% small bowel bypass was performed in diabetic rats (streptozotocin-treated) and in normal rats. Normal rats and diabetic rats were used as controls. Values of fasting blood glucose and oral glucose tolerance test showed a normalization and the disappearance of glycosuria, polyuria, polydipsia and hyperphagia in diabetic rats after surgery. Mean loss of weight 3 months after surgery was 9.1 % in normal bypassed rats and 60.5% in the diabetic controls. After an initial postoperative weigth loss of 33.4%, the diabetic bypassed rats gained subsequently their previous weight plus an increase of 7.2%. Improvement in carbohydrate metabolism appears to be independent of loss of weight and decrease in food intake in lean diabetic rats. Amelioration of diabetes after jejunoileal bypass is the result of several metabolic consequences, particularly the malabsorption of carbohydrates, fats and amino acids.