Glutamine may be a conditionally essential amino acid in low birth weight preterm neonates. An increase in the amount of exogenously administered amino acids, by providing anaplerotic carbon into the tricarboxylic acid cycle, could result in greater cataplerotic efflux and greater glutamine de novo synthesis. We hypothesized that an increased dose of administered intravenous amino acids during the immediate neonatal period when they are acutely stressed will result in increased de novo glutamine synthesis and urea synthesis and will not be utilized for protein synthesis. To test this hypothesis, preterm neonates (< 32 weeks gestation with birth weights from 809 to 1755 grams) were randomized within 48 hours after birth to initially receive either 480 or 960 µmol.kg^-1h^-1 (equaling 1.5 or 3.0 g.kg^-1.d^-1, respectively) of an intravenous amino acid solution (containing no glutamine) for 19 to 24 hours. The neonates then received the different amino acid load (480 or 960 µmol.kg^-1h^-1) for either a short (5 h) or extended (24 h) period. Glutamine de novo synthesis, leucine nitrogen kinetics, whole body proteolysis and irreversible nitrogen loss were determined using respective stable isotopic tracers. When the amino acid infusion was increased from 480 to 960 µmol.kg^-1.h^-1 for a short period of time (5 hours), glutamine de novo synthesis decreased in every neonate (384.4 ± 38.0 to 368.9 ± 38.2 µmol.kg^-1.h^-1, P<0.01). There was an associated decrease in whole body rate of proteolysis (P<0.001) and urea synthesis (P<0.001). However, when the increased amino acid infusion was extended for 24 hours, glutamine de novo synthesis increased (369.7 ± 92.6 to 483.4 ± 97.5 µmol.kg^-1.h^-1, P<0.001), whole body rate of proteolysis did not change, and the rate of urea production increased. Decreasing the amino acid load resulted in a decrease in glutamine Ra, leucine N Ra, but had no effect on phenylalanine Ra. Contrary to our hypothesis, even acutely stressed LBW infants responded to an acute increase in amino acid load by suppressing whole body rate of proteolysis and oxidation of protein. However, such a response was not sustained and continued infusion of higher amino acid load resulted in a higher rate of glutamine and urea synthesis. The mechanisms of the transient effect of amino acids on whole body protein/nitrogen metabolism remain unknown.