To investigate intestinal and hepatic metabolism of phenylalanine, four conscious pigs (7.5 kg), bearing arterial, venous, and hepatic portal catheters, were fasted for 12 h and infused with [phenyl-2H5]phenylalanine via a peripheral vein and [carboxyl-13C]phenylalanine via the stomach. During the first 6 h of the infusion, the pigs remained fasted and received only the intravenous tracer. During the second 6 h, they received an intragastric infusion of milk replacer and both tracers. In the fasted state, the portal-drained viscera extracted 10% (P < 0.025) of the arterial [2H5]phenylalanine flow of the pigs. In the fed state, the splanchnic tissues metabolized 45% of the enteral tracer and intestinal metabolism accounted for 76% of the total splanchnic extraction. The tracer-to-tracee ratio of both tracers in apolipoprotein B-100 (apo B-100) phenylalanine was twofold (P < 0.001) higher than that of hepatic free phenylalanine. The ratios of the two tracers in portal (13C/2H; 1.66) and apo B-100 (1.76) phenylalanine were similar but higher (P < 0.05) than that of arterial phenylalanine (1.29). We conclude that intestinal metabolism dominates the splanchnic extraction of enteral phenylalanine and that in the fed state, the hepatic protein synthetic precursor pool derives from portal phenylalanine.