An experiment was carried out to determine the flux of amino acids across the portal vein-drained tissue into and out of the plasma and red blood cell free amino acid pools and the plasma peptide pool of the portal vein blood of growing pigs. Four pigs, fitted with catheters in a carotid artery and the portal and mesenteric veins, were fed a maize starch-based diet with wheat gluten as protein source. The feed was given daily at the level of 2% body weight as a wet mash (1:2, w/v water) in two equal meals at 08.00 and 16.00 h. After 5-d adaptation to diet, the pigs received their 08.00 h meal and, subsequently, were not fed again for 28 h. At 4 (fed) and 28 (unfed) h after receiving the meal, simultaneous 8 mL blood samples were taken from the portal vein and carotid artery of each pig. The flow of whole blood and plasma were determined by the indicator-dilution method, using continuous infusion of p-aminohippuric acid into the mesenteric vein. Blood sampling and flow measurements were repeated 5 d later. Total flux of amino acids were higher (P=0.02) in the plasma (55.7 vs 1.9 mmol/h) and plasma peptide (123.2 vs -1.2 mmol/h) pools of portal vein blood of fed compared to unfed pigs, respectively. Corresponding total flux in the red blood cell pool (77.5 vs 5.9 mmol/h) was not statistically significant (P=0.29) because of a large standard error due to inconsistency between the pigs and accumulated analytical variation. A substantial proportion of the amino acids in the plasma peptide pool of portal vein blood of fed pigs were of dietary origin based on a similar amino acid profile to that of wheat gluten. In conclusion, the plasma free amino acid and plasma peptide pools, and probably the red blood cell free amino acid pool, of portal vein blood are involved in the flux of amino acids across the portal vein-drained tissue of pigs.