Amino acids in rat systemic and portal vein plasma and jejunal and ileal gut contents after 7 days of feeding normoprotein (NP) and protein-free diet (PF) are investigated. Amino acid analyses revealed that ingestion of PF diet resulted in unusual amino acid patterns in both plasmas. Thus, while the levels and/or molar ratios of all indispensable amino acids were significantly decreased, those of several gluconeogenetic amino acids, especially of glycine and alanine, were increased in both plasmas, but particularly in portal. By contrast, the molar ratios of the majority of amino acids in jejunal and ileal contents were not changed by PF diet.