To test Elwyn's suggestion [1970], that high concentrations of amino acids supplied to the liver from the hepatic artery do not stimulate protein synthesis, substrates containing amino acids have been infused into either the hepatic artery or portal vein of isolated sheep livers. The livers received highly oxygenated blood from the hepatic artery and partly deoxygenated blood from the portal vein. There were no significant differences in amino acid uptake (81 +/- 4% of input), urea output (69 +/- 6% of uptake) or 'protein synthesis' as assessed by N accumulation in the liver. Amino acids were actively extracted from the plasma by both routes of infusion and , when the concentrations simul-ted those occurring in the portal vein of fed sheep, the uptake was very similar to that in vivo. When an amino acid mixture based on that absorbed in dogs was infused, the extraction of individual amino acids was similar except for a negligible uptake of valine, leucine and isoleucine. In these experiments protein synthesis was also very low.