To investigate the utilization of dietary amino acids for hepatic protein synthesis, seven female pigs ( 28 d old, 7.5 kg) were implanted with catheters in a carotid artery, the jugular and portal veins, and the stomach. A portal flow probe was also implanted. The pigs were fed a high protein diet once hourly and infused intragastrically with [U-13C]algal protein for 6 h. Amino acid labeling was measured in arterial and portal blood, in the hepatic free and protein-bound pools and in apolipoprotein B-100 (apoB-100), albumin and fibrinogen. The isotopic enrichments of apoB-100-bound [U-13C]threonine, leucine, lysine and phenylalanine were 33, 100, 194 and 230% higher than those of their respective hepatic free amino acid pools (P < 0.01). Using the labeling of apoB-100 to estimate that of the protein synthetic precursor, the fractional rate of hepatic protein synthesis was 42 +/- 2%/d. Between 5 and 8% of the dietary tracer amino acids was used for hepatic protein synthesis. In contrast to the small intestinal mucosa, in which the majority of the metabolized amino acids were apparently catabolized, protein synthesis utilized from 48% (threonine) to 90% (lysine) of the hepatic uptake of tracer amino acids. It appears that hepatic protein synthesis consumes nutritionally significant quantities of dietary essential amino acids in first pass and that extracellular, especially portal, essential amino acids are channeled to hepatic protein synthesis in the fed state.