A single meal of arginine-free diet caused severe hyperammonemia in young (2-month-old) jerrets, whereas adult (18-month-old)Jerrets did not develop hyperammonemia after identical treatment. To explore the mechanism of hyperammonemia indueed by arginine-free diet. we tested the effect of a single meal of arginine-J~'ee diet on the amino acid levels in plasma, liver, and kidney of young and adult ferrets. Plasma and tissues were obtained three hours triter feeding the diets. Arginine-containing diet caused a rapid increase in plasma arginine and ornithine (compared to fasting levels) only in young ferrets, indicating that these amino acids were absorbed more rapidly in young ferrets than in adult ferrets. Young ~forrets fed arginine-Jkee diet had lower levels ยข~f plasma arginine titan those fed arginine-containing diet, whereas in adult ferrets no differences were observed in plasma arginine levels between arginine-free and arginine-containing diet groups. Both young and adult Ji~rrets had low levels of hepatic ornithine. However, only young Jerrets became hyperammonemic aJier ingesting,, an arginine-free diet. A decreased synthesis or an increased catabolism of arginine in young J~wrets may explain the rapid development of hyperammonemia resulting from ingestion of an arginine-free die t.