Insulin secretion and insulin action were studied in rats fed either a diet containing (as percent of calories) 66% fructose, 22% protein, and 12% fat, or standard rat chow (60% vegetable starch, 29% protein, 11% fat) for 7 days. Plasma glucose concentration following either an oral glucose or fructose load (180 mg/100 g body weight) were slightly higher in the fructose-fed rats, and this was associated with a much greater elevation of plasma insulin concentrations. The ability of insulin to stimulate disposal of glucose load was determined during the continuous infusion of epinephrine, propranolol, glucose, and insulin. Under these condtions the steady state plasma insulin levels were the same in the two groups of rats, whereas the steady state plasma glucose levels were almost twice as high in the fructose fed rats. Thus, fructose feeding for 7 days resulted in an increase in the insulin response to an oral carbohydrate challenge, as well as to a loss of normal insulin sensitivity.