Low plasma renin activity (PRA) has been reported in patients with long-term diabetes mellitus complicated by hypertension and nephropathy. We have assayed PRA in twelve normal subjects and in eight age- and sex-matched juvenile diabetics of greater than twelve years' duration without hypertension and nephropathy under control conditions and following stimulation with diazoxide. During control conditions PRA did not decrease with time in the diabetics as it did in the normals. Following diazoxide infusion, PRA increased in both groups, and although the levels were often higher in diabetics than in normals, the mean differences were not statistically significant. The findings are consistent with the suggestion that PRA is normal or possibly elevated in clinically uncomplicated insulin-dependent diabetes mellitus and decreases with establishment of hypertension and nephropathy.