To investigate the extent of first-pass intestinal metabolism of dietary amino acids, seven female pigs (28 d old, 8.0 kg) were implanted with arterial, venous, portal and gastric catheters and with an ultrasonic portal blood flow probe. The pigs were fed a milk-based diet once hourly and infused intragastrically with [U-13C]algal protein. On average, 56% of the essential amino acid (EAA) intake appeared in the portal blood. However, the net portal balance of methionine (48% of intake) and threonine (38% of intake) tended (P = 0.08) to be lower than the mean of all EAA. The net portal balance (expressed as a percentage of intake) of alanine (205%), tyrosine (167%) and arginine (137%) exceeded their intake. Net portal outflow of ammonia accounted for 18% of total amino acid nitrogen intake. As a percentage of the enteral tracer input, there was substantial first-pass metabolism of lysine (35%), leucine (32%), phenylalanine (35%) and threonine (61%). However, only 18, 21, 18 and 12% of the total first-pass metabolism of lysine, leucine, phenylalanine and threonine, respectively, were recovered in mucosal protein. We conclude that roughly one third of dietary intake of EAA is consumed in first-pass metabolism by the intestine and that amino acid catabolism by the mucosal cells is quantitatively greater than amino acid incorporation into mucosal protein.