The long term effect of dietary carbohydrate content on the course of noninsulin-dependent diabetes has been assessed in rats with experimental chemical diabetes (CD) obtained as spontaneous evolution of neonatal streptozotocin acute diabetes. Glucose tolerance and insulin secretion were serially tested before and during high carbohydrate diet (2 and 6 mo) or high lipid diet (1 mo) in control and in CD males. In none of the control or CD groups, did the high sucrose diet significantly affect the weight gain as compared to that obtained upon the standard diet. The high sucrose diet increased the insulin response to glucose and slightly improved the glucose tolerance in the normal rats. In CD rats, it increased the insulin secretion and the insulinogenic index; glucose tolerance was unaffected. In control rats receiving the high lipid diet, the weight gain was significantly increased as compared to that obtained upon the standard diet. Their plasma insulin levels were increased both in basal and glucose stimulated states while glucose tolerance remained unchanged. In the CD rats receiving the high lipid diet, weight gain was significantly increased as compared to that obtained upon the standard diet. But in these CD rats, the insulin secretion was not significantly enhanced by the high fat diet and the glucose tolerance deteriorated. These findings indicated a beneficial long-term effect of high carbohydrate diet in the CD rats as far as their insulin response to glucose was concerned. By contrast high lipid diet may be regarded as an aggravating factor of glucose handling in chemical diabetes. The results are discussed with regard to; (1) the increased insulin production and/or release on the high sucrose diet, and (2) the decreased insulin sensitivity on the high lipid diet.