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 (HbA1: N = 7.6 +/- 1.3%, N = 8.0 +/- 2.2%; normal less than 6.0%). Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured using the clearances of 125I-thalamate and 131I-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 m2 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%; % delta GFR vs. % delta ERPF p less than 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 less than 0.01; M: BL FF 0.25 +/- 0.01 vs. S FF 0.27 +/- 0.01, p less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)