We investigated the effects of hypotonic saline-induced modifications of extracellular volume and sodium handling on the renal and metabolic response to amino acids (AA). Renal hemodynamics (Inutest, p-aminohippurate clearance), plasma AA, and glucagon levels, as well as urea and sodium excretion, were studied in seven adult volunteers infused for 2 h, on six separate occasions, according to the following protocols: 1) high-AA solution (300 mg · min¹ · 1.73 m²); 2) low-AA solution (150 mg · min¹ · 1.73 m²); 3) low AA + 2,000 ml/1.73 m² of 0.23% saline solution; 4) high AA + 0.23% saline; 5) high AA + 0.45% saline; and 6) 0.45% saline alone. The glomerular filtration rate (GFR) rise induced by the high-AA solution was similar to that induced by the low-AA solution (GFR = +24 ± 6 and +20.2 ± 7 ml · min¹ · 1.73 m², respectively), whereas the plasma AA and glucagon levels and urea excretion rate increases were related to AA dose. The addition of 0.23% saline to the low-AA solution and of 0.45% saline to the high-AA solution blunted the renal hemodynamic response (GFR = +6.6 ± 10.1 and +11.4 ± 8.3 ml · min¹ · 1.73 m², respectively) without modifying the pattern of plasma AA and glucagon levels and urea excretion observed with the AA infusion alone. Urinary sodium excretion increased from baseline with each protocol and rose even further when saline was added to AA. A negative correlation (r = 0.38, P < 0.05) was found between the changes from basal values in GFR and those in sodium excretion rate with high-AA infusion at different levels of sodium concentration. These data suggest that AA-induced hyperfiltration might be blunted by hypotonic saline infusion, possibly through an acute modification of renal sodium handling and extracellular volume.