BACKGROUND The purpose of this study was to quantify the kinetic aspects of leucine metabolism in the whole body and within the splanchnic region when an amino acid mixture is administered by the enteral route as compared with administration by the parenteral route. METHODS Seven chronically catheterized dogs were studied during an intragastric infusion of L-[1-13C,15N]leucine with 0.24 g of complete amino acid mixture (kg-1.h-1) for 6 hours. The results are compared with those previously reported for nine dogs studied under essentially identical experimental conditions except that the tracer and amino acid mixtures were given by vein. RESULTS At the whole body level, the various parameters of leucine metabolism (flux, oxidation, and disappearance into and release from proteins) were not significantly different among animals that were infused enterally and those that were infused parenterally. Leucine metabolism within the splanchnic region and gut was more extensive for enteral administration than for parenteral administration. For the splanchnic region, 31.4%, 27.9%, and 6.0% of enterally administered leucine was used for protein synthesis, deamination, and oxidization, respectively, compared with respective mean values of 19.5% (p < .001), 13.4% (p < .001), and 4.1% (p < .05) for parenterally administered leucine. For liver, 4.8% of enterally administered leucine was oxidized compared with 2.3% (p < .001) of parenterally administered leucine. These findings are qualitatively similar to those reported previously by us when comparing enteral and parenteral amino acid intakes but with an IV tracer infusion. CONCLUSIONS Enteral administration seems to be more effective than an acute parenteral feeding regimen, at least in maintaining leucine (protein) balance in gut tissues.