To investigate plasma renin and prorenin levels in non-insulin-dependent diabetes mellitus (NIDDM) and their relation with autonomic nervous function and renal impairment, we measured plasma renin and prorenin levels in 39 NIDDM patients. The patients included 21 males and 18 females, aged 56.3 +/- 6.2. Thirty-four normal age-matched subjects served as controls. Autonomic nervous function was evaluated in 23 patients by the performance of cardiovascular reflex tests. The plasma renin concentration was measured by angiotensin I generation after the addition of an exogenous substrate. Plasma prorenin was activated by trypsin. The results showed that the plasma renin concentration was similar between NIDDM patients and normal subjects, while plasma prorenin was higher in NIDDM patients. No correlation existed between the plasma renin or prorenin levels and autonomic nervous function. The patients with abnormally high levels of prorenin also had a similarly high plasma renin level but not a high creatinine clearance (Ccr) or daily proteinuria. The plasma renin level was correlated inversely with daily proteinuria but not with Ccr. These results suggest that the high plasma prorenin levels in some diabetic patients cannot be explained by renal impairment, poor prorenin conversion or autonomic dysfunction. The hyporeninemia in some patients may be related to microvascular involvement of the kidney.