We have studied the effects of increased dietary carbohydrate content on glucose transport and intracellular glucose metabolism in adipocytes from spontaneously obese rats and from streptozotocin-induced insulin-deficient diabetic rats. Four groups of animals were studied: 1) obese rats on the control diet, 2) obese rats on the high carbohydrate diet, 3) diabetic rats on the control diet, and 4) diabetic rats on the high carbohydrate diet. Compared to the control diet, the high carbohydrate diet led to an increase in insulin secretion in the obese rats, whereas the diabetic animals were unable to respond to the diet with enhanced insulin output. In the diabetic animals, the rates of adipocyte glucose transport, glucose oxidation, and lipogenesis were low relative to those in nondiabetic controls and were not influenced by the high carbohydrate diet. In the obese animals on the control diet, absolute rates of glucose transport were similar to those of lean controls, but glucose oxidation and lipogenesis were depressed. On the high carbohydrate diet, all aspects of cellular glucose transport and metabolism were markedly increased. Thus, when plasma insulin levels were allowed to increase (obese animals), rates of cellular glucose metabolism increased; when plasma insulin was kept constant (diabetic rats) cellular glucose metabolism was unchanged. In conclusion, 1) a high carbohydrate diet leads to augmented rates of glucose transport, oxidation, and lipogenesis, provided the animal can respond to the increased dietary carbohydrate with increased insulin secretion, and 2) the diet-induced relative hyperinsulinemia most probably mediates the changes in cellular metabolism.