In hypertensive humans and the spontaneously hypertensive rat, increased cellular Na+/H+ antiport activity has been demonstrated in leukocytes, platelets, skeletal muscle, and vascular smooth muscle cells. This membrane abnormality may be associated with medial thickening of resistance vessels. A similar membrane transport abnormality has also been demonstrated in leukocytes and fibroblasts from type 1 diabetic patients with nephropathy. This membrane transport marker of hypertension may indicate a predisposition to essential hypertension in such patients and may lead to diabetic nephropathy, possibly from mesangial expansion.