Nitrogen metabolism and plasma insulin level were studied postoperatively in 14 patients (six males and eight females) with a disease of the upper gastrointestinal tract and therefore operated on electively. The patients received one of the two isocaloric parenteral nutrition regimens postoperatively: one, on the average, with 1.2 g of amino acids/kg/day and the other with 3.1 g of amino acids/kg/day. During postoperative intravenous alimentation rich in amino acids the cumulative nitrogen balance over 3 days was +13.1 (interval from -1.3 to +21.4) gN but -10.1 (interval from -12.1 to -2.4) gN during parenteral nutrition with a smaller amount of amino acids. The difference was significant (p less than 0.001). During parenteral nutrition rich in amino acids the changes of the serum albumin level, ie, -0.4 (SEM 1.1) g/liter, and of the serum transferrin level, ie; -0.16 (SEM 0.22) g/liter, were statistically insignificant (p greater than 0.05). During intravenous alimentation poor in amino acids serum albumin decreased by 3.8 (SEM 1.2) g/liter (p less than 0.01) and serum transferrin by 0.44 (SEM 0.05) g/liter (p less than 0.001). The differences of the changes between the groups were significant (p less than 0.01 and p less than 0.01, respectively). These various effects of the two parenteral nutrition regimens were not dependent on the different fluid balances during intravenous alimentation or on the different plasma insulin levels. It is concluded that a rich supply of amino acids--more than 1.2 g/kg/day--in postoperative parenteral nutrition better maintains the visceral protein levels in the serum, which possibly depends on the greater protein production in the liver.