This work attempts to determine if there are differences in protein metabolism in post-surgical patients who receive parenteral nutrition with amino acids plus glucose (G+AA) or conventional gluco-salinal solution (GS). Eighteen patients submitted to gastrointestinal surgery were randomized and double-blindly administered either G+AA (1 g AA/kg x d and 28 kJ/kg x d), or GS (28 kJ/kg x d). Protein metabolism was determined 12 h after surgery (day 0) and after 5 days of nutritional support. On day 0, protein breakdown was similarly elevated, with respect to reference values, in both groups (GS: 4.62 +/- 0.25; G+AA: 5.25 +/- 0.50 g prot/kg x d) as a result of surgical stress. These values increased significantly at day 5 (P < 0.03) with the administration of GS to 6.93 +/- 1.00 g prot/kg x d, while they decreased (P < 0.002, 3.30 +/- 0.42 g prot/kg x d) with G+AA. Protein synthesis was increased (5.69 +/- 0.86 g prot/kg x d) with GS (P < 0.02), and was decreased (2.79 +/- 0.44 g prot/kg x d) with G+AA (P < 0.0002). Both synthesis and breakdown were inside normal reference values after 5 days for group G+AA. In both groups, nitrogen balance did not change significantly at day 5 compared to day 0. G+AA is effective in curbing the hypermetabolism produced by postoperative stress, achieving normal protein metabolism in 5 days, while GS increases the protein breakdown and synthesis. Nitrogen balance does not detect these modifications of the protein metabolism. Undernutrition on prognosis is not yet fully recognized.