The mechanism of lowered renin-aldosterone system was investigated in 17 patients with diabetic nephropathy (serum Cr < 3 mg/l00 ml) with concomitant control of the blood sugar level. The response of plasma renin activity (PRA) to upright stimulation was lower in the low renin group (group I) than in the normal to high renin group (group II) and in the control group. The PRA response to theophylline was delayed in group I. The percentage of the luminal area of the arteriole in the biopsy specimens was larger in group I and the control group than in group II. Plasma aldosterone concentration (PAC) was not increased by angiotensin II in group I. Low PRA in diabetic nephropathy with slightly to moderately impaired renal function may not be due to hyaline destruction of the arteriolar walls, but to other factors such as sympathetic nervous dysfunction. The adrenal responses of PAC to angiotensin II may also be impaired.