The use of total parenteral nutrition after acute injury, either surgical or accidental, is widely accepted for its important benefits, although it is not yet completely understood whether a reduction of body protein catabolism can be effectively achieved. We applied total parenteral nutrition to 14 critically ill patients after either trauma or major surgery. Their daily nitrogen balance, urinary 3-methylhistidine and creatinine excretion, and molar 3-methylhistidine/creatinine ratio, during initial 24-hr fasting period, were respectively -0.19 +/- 0.01 (SEM) g kg-1, 5.46 +/- 0.47 mumol kg-1, 27 +/- 4 mg kg-1, and 0.030 +/- 0.005. Daily nonprotein calorie intake of 31.11 +/- 0.58 kcal kg-1, as glucose, and administration of nitrogen 0.350 +/- 0.004 g kg-1, as 10% crystalline L-aminoacids solution, and insulin 1 IU every 5.03 +/- 0.14 g of glucose, resulted in progressive decline of urinary 3-methylhistidine (4.21 +/- 0.43 mumol kg-1, p less than 0.001), creatinine (22 +/- 2 mg kg-1, NS), and their molar ratio (0.022 +/- 0.002, NS). Mean nitrogen balance resulted in 0.032 +/- 0.008 g kg-1. Since urinary 3-methylhistidine role as a marker of protein catabolism is well established, its decrease under total parenteral nutrition together with greatly improved nitrogen balance, demonstrates that our treatment can effectively quench protein catabolism, meanwhile enhancing protein synthesis.