Oligopeptides have been proposed as an important form of transport of amino acids in circulation, but current preparation and analysis methods do not allow detection of peptides in plasma samples. In order to measure the concentration and composition of bound amino acids in comparison with those under free form in plasma, blood samples were collected in portal vein and aorta of rats fed a 20% rapeseed protein diet. Samples were deproteinized with sulfi)salicylic acid, and centrifuged in 3000 molecular weight cut-off filters. Analyses were performed by ion-exchange chromatography befi)re and after hydrolysis of filtrates in 6N HCI, to assess by difference the amount of bound amino acids in plasma. Hydrolyzed samples from portal vein and aorta contained, respectively, 8.2% and 10.3% more total amino acids than nonhydrolyzed samples (P < 0.05). In portal vein, this difference was mostly due to proline (31%), arginine (17%), and threonine (10%). In aorta, proline (40%), glycine (18%), and threonine (11%) accounted for most of the difference. In both vessels, no difference was noted before and after hydrolysis for serine, valine, tyrosine, and histidine, indicating that these amino acids were found exclusively under free form. These results show that in the fed animal, conjugates may account for around 10% of amino acid transport in portal and systemic plasma, and do not contain all amino acids in equal proportions. The dietar), or endogenous origin of these bound amino acids has not been established yet.