To evaluate the renin-angiotensin-aldosterone system in diabetes mellitus, basal plasma renin activity(PRA) and its response to intravenous furosemide were determined in 40 diabetic subjects. The diabetics were divided into 4 groups according to the pressence of nephropathy and/or hypertension. Uncomplicated diabetics(Group I) were taken as control group and the results of the other groups were compared to this group. In diabetics viith nephropathy alone(Group Ⅱ), and with nephropathy and hypertension (Group Ⅲ), basal PRA values were 0.63±0.59ng/ml/hr., and 0.79±0.62ng/ml/hr., respectively, both significantly lower than control group. (1.53±1.09ng/ml/hr.). (P<0.05) In both of the above groups, the responses to intravenous furosemide tended to be blunted. On the other hand, in diabetics, with hypertension only (Group Ⅳ), the basal and stimulated PRA were not significantly different from control. Above results suggests that nephropathy may be one of the factors which suppress renin activity in diabetes mellitus.