Hypertension may eventually develop in response to chronic slight retention of sodium and expansion of the extracellular fluid volume, either due to intrinsic pathology or to neurohormonal influences of the kidneys. As almost half of all juvenile diabetic patients sooner or later will develop diabetic nephropathy and hypertension, data are discussed which tend to indicate that renal sodium metabolism is altered already early during the course of diabetes. Compared to healthy subjects the absolute total tubular sodium reabsorption is increased by approximately 30-40 per cent, as is the filtered sodium load. Insulin may stimulate sodium reabsorption in man through an effect on the distal nephron segment. However, by means of combined lithium and 51Cr-labelled EDTA clearances it has been clearly demonstrated that the excess sodium reabsorption in ambulatory insulin-dependent diabetics exclusively takes place in the proximal tubules, while the distal tubular function appears normal. In these studied patients the extracellular fluid volume was also significantly increased. The increased fractional sodium reabsorption of the proximal tubules remains unaffected by increasing duration of diabetes and is also demonstrable in patients with overt diabetic nephropathy. Glucose is reabsorbed in the early portion of the proximal tubules coupled to Na+ transport, utilizing a common carrier protein. An increased load of glucose will therefore be expected to induce an increase in the proximal tubular reabsorption rate of sodium and water, at least as long as the proximal tubular reabsorption capacity for glucose is not exceeded to a degree inducing significant osmotic diuresis. This deviation from normal in proximal renal sodium and fluid handling may be relevant to the development of hypertension in long-term insulin-dependent diabetes.