Diabetes mellitus is associated with important changes in renal hemodynamics and transport function. Disturbances in solute transport have also been characterized in nonrenal tissues during hyperglycemia and insulinopenia. The purpose of this study was to determine if diabetes is associated with adaptive changes in function of the brush-border membrane of the proximal tubule. We studied Na+ and glucose transport in rat microvillus membrane vesicles isolated from the renal cortex of streptozotocin-induced and BB/W autoimmune diabetic rats. Untreated diabetes was associated with an increase in pH-stimulated total and amiloride-sensitive 22Na+ uptake into vesicles. Insulin treatment returned vesicle 22Na+ uptake to control levels. The increased Na+/H+ exchange was shown to be a result of increased net renal acid production rather than a specific response to insulinopenia because treatment with NaHCO3 also returned 22Na+ uptake to control levels. On the other hand, Na+-glucose cotransport, which was depressed in vesicles from untreated diabetics, returned to control levels with insulin but not NaHCO3 administration. This decreased Na+-glucose cotransport was not secondary to reduction in transport sites in untreated diabetics. These results show that in diabetes mellitus, increased Na+/H+ exchange activity is not the direct result of insulinopenia. However, the diabetic state appears to alter the functioning of the luminal Na+-glucose cotransporter.