Renal reserve filtration capacity (RRFC) was investigated in 20 kidney donors before and within 3 months after uninephrectomy. Glomerular filtration rate (GFR) was measured by the clearance of 125I-iothalamate, and effective renal plasma flow (ERPF) was determined by the clearance of 131I-hippurate. RRFC was tested by infusion of a low dose of dopamine, by intravenous administration of an amino acid solution and, finally, by combined infusion of amino acids and low-dose dopamine. After kidney donation, the median value of the GFR was 65% of its initial value. The median value of the ERPF was 69% of its value before uninephrectomy. The filtration fraction declined significantly from 0.251 to 0.238 (medians; P less than 0.02). Median values for dopamine-induced increases in GFR were 13.8% before and 5.3% after kidney donation (P less than 0.005). Infusion of amino acids led to an increase in GFR of 11.2% before and 9.6% after donation (NS). During combined infusion, the median values for the increases in GFR were 20.0% before and 12.6% after uninephrectomy (P less than 0.02). Median changes in ERPF before and after kidney donation were 33.4 vs. 23.0% during dopamine infusion (P less than 0.005), 10.0 vs. 8.3% during amino acid infusion (NS), and 36.2 vs. 24.3% during the combined infusion of dopamine and amino acids (P less than 0.02). Infusion of dopamine induces a decrease in renal vascular resistance, resulting in a rise in glomerular blood flow. Since RRFC tested by means of dopamine infusion was significantly decreased after kidney donation, it is concluded that glomerular hyperfiltration after kidney donation is at least partially due to a rise in glomerular blood flow.