Rates of protein turnover were measured in 20 infants receiving either Vamin Infant (group A) or Vamin 9 glucose (group B) as the amino acid source in total parenteral nutrition. A constant infusion of L-[1-13C]leucine was used to measure whole body leucine flux, and leucine oxidation rates were derived from measurements of total urinary nitrogen excretion. Infants were first studied when receiving only i.v. glucose and again on each of the next 4 d as total parenteral nutrition was gradually increased to a maximum of 430 mg nitrogen/kg/d and 90 nonprotein kcal/kg/d. Net protein gain and protein synthesis and breakdown rates increased progressively for all infants taken together over the study period as i.v. nutrition was increasing (p less than 0.001). There were no differences between groups in the changes in net protein gain and rates of protein synthesis and breakdown throughout the study period. Nitrogen retention on d 5 for the two groups was similar (60 +/- 16% and 67 +/- 11% in groups A and B, respectively). In a subgroup of infants, measurements were repeated on d 8, when the intake had been constant for 3 d. Protein retention was the same as on d 5, but both synthesis and breakdown were increased. It is concluded that rates of protein turnover increase significantly in response to increasing i.v. nutrition and that this elevation was not influenced by the composition of the amino acid mixture given.