Background: Muscle glutamine is severely depleted in critically ill patients (by ~50% to 80% of normal). Because muscle protein breakdown, and thus the release of glutamine from muscle protein, is enhanced in response to metabolic stress, the depletion of intramuscular glutamine could be due to its impaired synthesis or accelerated outward transport or both. Methods: To distinguish these possibilities, we measured skeletal muscle glutamine metabolism in five critically ill patients by means of primed, continuous infusions of 5-15N-glutamine and ring-2H5-phenylalanine and compared them to values we previously reported for healthy volunteers. Results: The intramuscular free glutamine concentration in patients was ~70% of that in healthy volunteers (5.8 ± 0.6 mmol/L intracellular free water vs 21.5 ± 2.8 mmol/L). Whole-body glutamine rate of appearance was 5.8 ± 1.0 μmol · kg- 1 body wt · min-1, and whole-body clearance was 19.3 ± 3.3 mL · kg-1 · min-1. Despite the low intramuscular glutamine concentration in the patients, the rate of unidirectional outward transport from skeletal muscle into venous blood (1.1. ± 0.2 μmol · 100 mL · leg-1 · min-1) was similar to that observed in healthy volunteers (1.6 ± 0.2 μmol · 100 mL · leg-1 · min-1); intramuscular synthesis was 2.7 ± 0.9 μmol · 100 mL · leg-1 · min-1 compared with a normal value of 0.6 ± 0.06 μmol · 100 mL · leg-1 · min-1. Net balance across the leg was normal. Conclusions: The depletion of intramuscular glutamine in critically ill patients is not due to an impairment of the rate of synthesis. In fact, accelerated glutamine production cannot maintain normal intramuscular glutamine levels because of accelerated outward transport.