An effect of insulin on glycosuria independent of its effect on blood sugar was demonstrated in a group of seven diabetics. To do this it was necessary to develop a technique in which the hypoglycaemic effect of insulin could be isolated and overcome. This was achieved by monitoring the arterial blood glucose continuously and infusing glucose intravenously as necessary to maintain the arterial concentration. The technique may be useful in other investigations. The significance of the results is discussed. This is the first time that such an effect of insulin has been demonstrated, and it suggests that present theories on the renal handling of glucose are inadequate.