Studies were made on the relative contributions of amino acids for hepatic gluconeogenesis in vivo and on inter-organal relations in amino acid metabolism in fasted rats. First, the plasma levels of lactate and amino acids were determined 2 hr after evisceration-nephrectomy of fasted rats. The plasma levels of lactate, glutamine, alanine, glycine, and lysine increased remarkably, but those of aspartate, glutamate, valine, leucine, and isoleucine did not increase appreciably. Second, arterio-venous-differences in plasma amino acids across various tissues were determined. The-arterial-common iliac vein difference indicated that peripheral tissues, including the skeletal muscle, skin, and adipose tissue, released considerable amounts of glutamine, alanine, and glycine. The arterial-portal vein difference indicated that, non-hepatic splanchnic organs released much alanine plus a little glycine and glutamate, and utilized much glutamine plus a little serine. The arterial-renal vein difference indicated that the kidney produced much serine and utilized much glutamine plus a little glycine. The levels of acidic and branched-chain amino acids did not change appreciably across these organs. The portal-hepatic vein difference indicated that glutamine was released from the liver, and that other amino acids plus lactate were utilized by the liver in proportions similar to those of amino acids accumulated in. the plasma of eviscerated-nephrectomized rats. The significance of these findings is discussed. The results support the concept that alanine contributes most to hepatic gluconeogenesis in fasted rats, while the contributions of serine, glutamine, and other amino acids are far less. The results, also suggest that glutamine and alanine are “ end products” of amino acid metabolism in many tissues and act as “ nitrogen carriers” from organ to organ. From these findings, the inter-organal relations in amino acid metabolism in fasted rats were inferred and quantification of the relative contributions of individual organs in amino. acid metabolism was discussed.