Parenteral nutrition contributes considerably to the survival of operated newborns. The postoperative period, however, gives rise to special metabolic problems. Commercially available L-amino acid (L-AA) solutions may lead to imbalances in the postoperative period. We used a preparation containing all essential and non essential L-AA, which should take the stress metabolism into account. Twenty newborn infants were treated with a standardized parenteral nutrition regime after major surgery. Two comparable groups were formed; the first (A) receiving up to 2 g L-AA/kg/day and the second (B) 1.5 g L-AA/kg/day at the most. On the seventh postoperative day group B was loaded with 1 g L-AA/kg within 6 h corresponding to the transfer rate in healthy newborn infants calculated by Bürger and Wolf. Plasma L-AA were checked on the seventh day before and after loading. The serum L-AA showed almost normal values in group A, however, several values were found to be too low in group B (proline, alanine, cystine, leucine). After the L-AA load some increased above normal levels (threonine, glycine, lysine, histidine). With reference to these results a daily application of 1.5-2.0 g of L-AA/kg can be recommended for critically ill newborns in the early postoperative period.