We have recently shown that the requirements of several amino acids differ substantially when neonates are fed parenterally as opposed to enterally. Our first objective was to determine whether the tryptophan requirement was different in parenterally fed (IV(fed)/IV(dose)) versus enterally fed (IG(fed)/IV(dose)) piglets. Because of the extensive extraction of amino acids by the gut, our other objective was to determine whether the route of isotope administration [i.e. intragastric (IG(fed)/IG(dose)) versus i.v. (IG(fed)/IV(dose)) dose] affects the estimate of tryptophan requirement in enterally fed piglets. We used the indicator amino acid oxidation technique in piglets (10 +/- 0.5 d old, 2.79 +/- 0.28 kg) receiving a complete elemental diet for 6 d either intragastrically or intravenously. Piglets were randomly assigned to receive test diets containing one of seven levels of tryptophan. All animals received a primed, constant infusion of l-[1-(14)C]phenylalanine either parenterally (IV(fed)/IV(dose) and IG(fed)/IV(dose)) or enterally (IG(fed)/IG(dose)). The mean tryptophan requirements for IV(fed)/IV(dose) (0.145 +/- 0.023 g/kg/d), IG(fed)/IV(dose) (0.127 +/- 0.022 g/kg/d), and IG(fed)/IG(dose) (0.113 +/- 0.024 g/kg/d) were similar as were the safe intakes (upper 95% confidence interval) (0.185, 0.164, 0.154 g/kg/d, respectively). These data indicate that tryptophan is not extensively used by the gut, in contrast to all the other amino acids we have studied. Furthermore, in spite of a splanchnic extraction of 27% of the phenylalanine dose, the route of isotope infusion does not affect the tryptophan requirement as determined by indicator amino acid oxidation.