We evaluated the role of obesity in proteinuria by treadmill exercising of simple obese subjects and non-obese subjects with non-insulin dependent diabetes mellitus in whom the albumin excretion rate at rest was within normal range. Non-obese healthy volunteers were studied as the controls. The fractional renal clearances of four endogenous proteins, albumin, IgG, IgG4, and beta2-microglobulin were measured before, during, and after treadmill exercise in 17 simple obese and 15 non-obese diabetic subjects, and in 21 normal subjects. Exercise increased the fractional albumin clearance in all groups. In diabetic subjects, the fractional IgG4 clearance also increased: fractional beta2-microglobulin clearance increased in normal controls and in diabetics. In obese subjects, the fractional clearances of albumin, IgG, and IgG4 were similar to those in normal controls, but fractional beta2-microglobulin clearance was significantly lower. These results suggest that enhanced microalbuminuria in obese subjects is probably of glomerular origin. In normal subjects and diabetics, exercise-induced microproteinuria is probably of both glomerular and tubular origin. Defect in the charge-selective barrier of the glomerular capillary wall has been implicated in diabetics. Thus some additional factors relevant to obesity must be taken into account in the consideration of the mechanism of microalbuminuria in diabetics with obesity.