The role of the renin angiotensin system for the regulation of kidney function in diabetes mellitus is uncertain. Results from studies in diabetic animals suggest that a reduced activity in this system contributes to the renal hyperperfusion and hyperfiltration in diabetes. The renal sensitivity to angiotensin II in diabetic patients is also unknown. Changes in renal hemodynamics were measured after infusion of two low doses of angiotensin II in ten young type 1 diabetic patients without complications and in ten healthy controls. The renin and angiotensin II levels were found to be the same in both groups. The baseline glomerular filtration rate was higher in the diabetics. During the highest angiotensin II dose, the 51Cr-EDTA and PAH clearance decreased 14 +/- 15 and 157 +/- 118 ml/min in the diabetics and 14 +/- 15 and 146 +/- 109 in the controls respectively. The changes in blood pressure and renal vascular resistance or sodium excretion did not differ between the groups. A malfunction of the renin angiotensin system is thus unlikely as a cause of the glomerular hyperfiltration in type 1 diabetes.