To determine whether neonates undergoing major abdominal surgical procedures in the first day of life could achieve a positive protein balance without protein toxicity in the immediate perioperative period by using parenteral amino acids and fentanyl analgesia. Newborns undergoing major surgery for gastroschisis in the first 24 hours of life (n = 13) were alternately allocated to immediate postoperative parenteral administration of 1.5 g/kg(-1)/day(-1) versus 2.5 g/kg(-1)/day(-1) amino acids. Protein balance was determined at 1.6 to 2.6 days postoperatively with both nitrogen balance and leucine stable isotope methodology. Statistical analyses were conducted with the unpaired t test and linear regression. Protein balance was significantly different in the 2 groups with both nitrogen balance and leucine stable isotope methodology. There was no evidence of protein toxicity as determined with blood urea nitrogen, creatinine, and ammonia concentrations. Neonates undergoing the metabolic stress of abdominal surgery shortly after birth are able to achieve a net positive protein balance with parenteral amino acid administration without evidence of protein intolerance.