The effects of exercise on glomerular permeability were investigated in 12 proteinuric insulin-dependent diabetic patients and in 12 healthy controls by measuring the fractional protein and dextran clearances at rest and after exercise. Exercise significantly reduced the glomerular filtration rate (GFR) and the renal plasma flow (RPF) and markedly increased the filtration fraction (FF) in both diabetics and controls. The fractional clearances of albumin and IgG increased significantly during exercise in diabetics. Exercise also significantly increased the fractional clearance of albumin in healthy controls. The changes in the fractional protein clearances correlated significantly with the changes in the FF. In diabetics the fractional dextran clearances of molecules with a radius greater than or equal to 4.8 nm were significantly elevated after exercise. This was not found in healthy controls. It is concluded that exercise increases glomerular permeability by influencing the renal haemodynamics. Probably partial depletion of negative charges on the glomerular capillary wall plays a role in exercise-induced proteinuria in both healthy and diabetic subjects. In addition, the altered glomerular permeability during exercise is associated with increased size of the filtering pores in diabetic nephropathy.