In healthy individuals and in patients with varying degrees of impaired renal function, glomerular filtration rate (GFR) and effective renal plasma flow were measured before and during infusion of an amino acid solution (Vamin N). GFR increased during amino acid infusion in healthy individuals while the filtration fraction (FF) remained constant. However, in patients with impaired renal function no significant changes in GFR were observed. The FF increased slightly. We conclude that amino acid infusion can increase GFR, possibly by utilization of 'dormant cortical nephrons' together with a rise in net ultrafiltration pressure of other filtrating glomeruli, both due to afferent vasodilatation. Thus, amino acid administration can be used to test the presence of reserve filtration capacity.