SummaryNet nitrogen retention (NNR) and rates of whole-body protein turnover (Q), synthesis, and breakdown (B) were measured in 24 intravenously fed premature infants, birthweight <1600 g, at the end of the first week of life. Four regimes were used: Amigen-glucose ± Intralipid; Vamin-glucose ± Intralipid. Mean protein intake was 2.7 g/kg/day. Mean energy intakes were 68 to 98 kcal/kg/day.Vamin was a better protein source (P< 0.01), evidenced by a higher NNR; 72 ± 2%,cf. 56 ± 4% at high-energy intakes. The high-energy intake also improved (P< 0.01) protein retention (NNR); 64cf. 50%. Infants receiving 2.9 g of Vamin (394 mg N)/kg/day and 85 kcal/kg/day of nonprotein intake retained nitrogen at intrauterine rates (282 ± 7 mg/kg/day).Diet had no effect on Q, synthesis, or B. However, the protein source had a significant effect (P< 0.01) on the fraction of N-flux coming from protein breakdown (B/Q); 71.1% for Vamin,cf. 77.1% for Amigen. Similarly, energy intake had a significant effect (P< 0.01) on the fraction N-flux utilized for protein synthesis (S/Q); 91.3% high energycf. 87.0% low energy. These results suggest that an increased energy intake improved N-retention by enhancing amino acid reutilization for protein synthesis, whereas a higher quality protein improved N-retention by limiting protein breakdown.