A study was made of carbohydrate digestibility and weight gain response in rats fed sucrose, glucose or fructose at varying levels in the diet. In a long-term experiment with older rats, sucrose-fed animals attained a greater mature weight than glucose and fructose-fed animals. In a digestibility study of shorter duration, animals fed sucrose and glucose diets had similar weight gains whereas fructose-fed animals showed a diminished weight gain response at higher levels of fructose in the diet. The digestible energy of glucose was observed to be significantly lower than that for either sucrose or fructose. Whereas the lower digestible energy of glucose may account for the lower weight gain on high glucose diets, a similar explanation is not valid for the response to fructose feeding. It would appear that an effect other than lowered feed intake or decreased digestibility is the cause of the lower weight gains observed on high fructose diets.