Renal function was studied in mice of different ages. In metabolic cage experiments, the renal electrolyte excretion was similar in young (n = 8; 5- to 7-wk-old) and adult (n = 6; 20- to 22-wk-old) CD-1 (ICR) BR mice, whereas spontaneous drinking volume and urinary flow rate were significantly higher in the adult compared with the young mice. Subsequently, the renal functional reserve was investigated by amino acid (AA) infusion (10%) in anesthetized young (n = 8) and adult (n = 6) mice. Because the body weight of adult mice was significantly higher than that of young animals, one group of adult mice (n = 8) received 12.5% AA to ensure that the dose of AA related to body weight was similar in both groups. Young animals constantly infused with Ringer solution served as time controls (n = 8). Glomerular filtration rate (GFR) at baseline was similar in each group. Because of AA, GFR significantly increased in young mice but not in both groups of adult animals, whereas in time controls GFR remained constant. Urinary flow rate and sodium excretion were elevated by AA in young and adult mice. We conclude that in CD-1 mice the first signs of age-related changes in kidney function concern alterations in renal hemodynamics, whereas renal tubular function appears to be preserved.