Glomerular filtration rate (GFR) and renal plasma flow (RPF) were measured by constant inulin and PAH infusion during euglycemia and intravenous dextrose-induced moderate hyperglycemia in seven insulin-dependent diabetics with persistently elevated GFR, seven diabetics with normal GFR, and in six normal control subjects. In euglycemia, RPF was higher and calculated renal vascular resistance (RVR) lower in the hyperfiltering than the normofiltering group (P less than 0.05 for both variables), but filtration fraction (FF) was similar in all groups. During hyperglycemia, mean GFR rose significantly from 157 +/- 20 to 174 +/- 30 ml/min/1.73 m2 (11.9%; P less than 0.05) in the hyperfiltering group only. There was no statistically significant change in mean GFR in the normofiltering diabetic (116 +/- 6 vs. 114 +/- 13 ml/min/1.73 m2) and the normal control groups (117 +/- 15 vs. 113 +/- 14 ml/min/1.73 m2). RPF and FF rose by 5.8% and 9.2%, respectively, in the hyperfiltering group only, with no change in the normofiltering or normal control groups. No change in RVR was found in any group. Total tubular sodium reabsorption was higher during euglycemia in the hyperfiltering diabetics (P less than 0.01), and rose significantly during hyperglycemia (P less than 0.05) in this group only. Overnight euglycemia did not remove the increased glomerular filtration and flow of hyperfiltering diabetics. Hyperglycemia further accentuated hyperfiltration by elevating renal plasma flow and filtration fraction.