Cocks were fed a protein-free diet supplemented with methionine plus arginine or glutamic acid for 25 days to investigate the effect of these amino acids on fecal and urinary nitrogen excretion. Addition of either methionine plus arginine or glutamic acid did not change the fecal nitrogen excretion. Methionine plus arginine supplementation reduced the urniary nitrogen excretion compared to the protein-free diet, whereas glutamic acid supplementation increased it. The reduced urinary nitrogen excretion resulting from supplementation with methionine plus arginine was mostly accounted for by a reduction in uric acid excretion. In the methionine plus arginine group, free amino acid analysis showed that free glutamine and glutamic acid content significantly decreased in liver while no differences were found in plasma. Since glutamine may play an important role in the formation of uric acid for chickens, the reduced amount of free glutamine and glutamic acid in the liver of cocks fed the diet suplemented with methionine plus arginine might account for the reduced excretion of uric acid, and therefore for the nitrogen sparing action of methionine plus arginine in chickens fed a protein-free diet.