Twenty-three normoalbuminuric (N) and 7 microalbuminuric (M) insulin-dependent diabetes mellitus (IDDM) patients were studied under (near) normoglycaemic conditions. They were reasonably well controlled during the period preceding the renal function test (HbA<sub>1</sub>: N = 7.6 ± 1.3%, M = 8.0 ± 2.2%; normal < 6.0%). Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured using the clearances of <sup>125</sup>I-thalamate and <sup>131</sup>I-hippuran, respectively. The renal reserve filtration capacity (RRFC) was tested by using a combination of a liquid mixed meal and an amino acid infusion. Blood glucose levels were kept as constant as possible throughout the testing procedure, both under baseline (BL) conditions and after stimulation (S). Under such (near) normoglycaemic conditions, no BL GFR values exceeding 150 ml/min/1.73 m<sup>2</sup> could be established. Furthermore, a RRFC could be established in all patients. Both groups showed a comparatively larger increase in GFR (N 13.0 ± 3.8%, M 10.8 ± 3.6%) than in ERPF (N 4.8 ± 7.0%, M 2.2 ± 5.8%; %Δ GFR vs. % ΔERPF p < 0.01), resulting in a higher filtration fraction (FF) during stimulation (N: BL FF 0.25 ± 0.03 vs. S FF 0.27 ± 0.03, p < 0.0l; M: BL FF 0.25 ± 0.01 vs. S FF 0.27 ± 0.01, p < 0.05). This suggests afferent vasodilation during stimulation in these (near) normoglycaemic, reasonably well-controlled IDDM patients, a situation comparable to that in non-diabetic subjects. Thus, reasonably well-controlled normoglycaemic N or M IDDM patients appear to have a (virtually) normal renal function, a normal renal vascular reactivity, and a normal RRFC. In our opinion, these results emphasize the importance of strict metabolic control in IDDM patients to prevent continuous afferent vasodilation and hyperfiltration.