During critical illness glutamine deficiency may develop. Glutamine supplementation can restore plasma concentration to normal, but the effect on glutamine metabolism is unknown. The use of growth hormone (GH) and insulin-like growth factor I (IGF-I) to prevent protein catabolism in these patients may exacerbate the glutamine deficiency. We have investigated, in critically ill patients, the effects of 72 h of treatment with standard parenteral nutrition (TPN; n = 6), TPN supplemented with glutamine (TPNGLN; 0.4 g · kg¹ · day¹, n = 6), or TPNGLN with combined GH (0.2 IU · kg¹ · day¹) and IGF-I (160 µg · kg ¹ · day¹) (TPNGLN+GH/IGF-I; n = 5) on glutamine metabolism using [2-¹⁵N]glutamine. In patients receiving TPNGLN and TPNGLN+GH/IGF-I, plasma glutamine concentration was increased (338 ± 22 vs. 461 ± 24 µmol/l, P < 0.001, and 307 ± 65 vs. 524 ± 71 µmol/l, P < 0.05, respectively) and glutamine uptake was increased (5.2 ± 0.5 vs. 7.4 ± 0.7 µmol · kg¹ · min¹, P < 0.05 and 5.2 ± 1.1 vs. 7.6 ± 0.8 µmol · kg¹ · min¹, P < 0.05). Glutamine production and metabolic clearance rates were not altered by the three treatments. These results suggest that there is an increased requirement for glutamine in critically ill patients. Combined GH/IGF-I treatment with TPNGLN did not have adverse effects on glutamine metabolism.