We studied the renin-angiotensin system (RAS) and atrial natriuretic peptide (ANP) after amino acid loading in order to elucidate whether or not glomerular hyperfiltration is mediated by these hormones. Eight healthy controls without kidney disease (grop 1), 8 renal graft recipients (group 2) and 8 patients after nephrectomy (group 3) werde studied. Furthermore, we investigated the influence of amino acid loading on RAS and ANP and glomerular filtration during acute application of captopril in healthy controls. Clearances in inulin (C<sub>In</sub>) and para-aminohippuric acid (C<sub>PAH</sub>) werde determined during an infusion of 0.9% saline and during amino acid loading. C<sub>In</sub> increased in healthy controls from 96.5 ± 3.3 to 111.9 ± 4.4 ml/min (p < 0.01), C<sub>PAH</sub> rose from 577 ± 28.7 to 618.9 ± 38 ml/min (p = NS). During the combined application of amino acid infusion and 25 mg captopril, C<sub>IN</sub> increased to 125.0 ± 16.4 ml/min/1.73 m<sup>2</sup> (p < 0.01) and there was no significant increase in C<sub>PAH</sub> which was 595 ± 77 ml/min/1.73 m<sup>2</sup>. There was no significant change in renal vascular resistance, but filtration fraction increased from 17 ± 1 to 19 ± 1% under amino acid infusion and increased further under the application of captopril to 21 ± 2% (p < 0.05). In renal graft recipients, C<sub>In</sub> had a tendency to increase during amino acid infusion from 78.4 ± 5.8 to 84.7 ± 6.5 ml/min (p = NS). In patients after nephrectomy, C<sub>In</sub> did not increase (84.5 ± 6 ml/min) but C<sub>PAH</sub> did from 345.7 ± 26.3 to 409 ± 24.1 ml/min (p < 0.05). In healthy controls, ANP was not increased during amino acid infusion: 80.5 ± 9.3 pg/ml as compared to saline infusion with 62.0 ± 10.6 pg/ml (p = NS). During the administration of amino acid infusion in renal graft recipients, ANP was not increased: 283 ± 62.7 as compared to 334 ± 115.7 pg/ml under saline infusion. A renal functional reserve could be shown in healthy persons, but this was grossly diminished in graft recipients and patients after unilateral nephrectomy. Renal functional reserve was maintained in healthy controls during the application of captopril. This investigation indicates that the increase in glomerular filtration rate seen in healthy persons is not mediated by RAS or ANP.