Nitrogen retention and rates of whole body amino nitrogen flux, protein synthesis, and breakdown were measured in 18 neonates during the 72 h immediately postsurgery. The infants were all intravenously fed with a crystalline amino acid source (Vamin), glucose, and Intralipid The infants were divided into two groups based on amino acid intake: either 2.3 SD 0.4 g or 3.9 SD 0.5 g/kg/ day. Nonprotein energy intakes were similar and averaged 81 kcal/kg/day. Group A (n=11) retained 145 SD 110 mg N/kg/day, whilst group B (n=7) retained 315 SD 93 mg N/kg/day (p<0.001). There were no differences seen in flux, synthesis or breakdown. However, group B had significantly higher net protein synthesis rates (synthesis breakdown) (p<0.01). The improved nitrogen utilization in group B was achieved principally by a reduction in endogenous protein breakdown. There were no differences between the two groups in urinary creatinine or 3-methylhistidine excretion. Since these two parameters reflect skeletal muscle turnover the differences between groups in nitrogen retention and turnover appear to be mediated through visceral protein