Two groups of 10-day-old miniature pigs were maintained on isocaloric and isonitrogenous total parenteral nutrition (TPN) regimens for nine days. One group received nonprotein energy as glucose, whereas the second group received a mixture of fat and glucose. The administration of the amino acid/glucose fuel mix resulted in higher plasma insulin but lower glucagon concentrations compared to the amino acid/glucose/fat mix. Differences also were observed in the composition of skeletal muscle, which contained higher concentrations of alkali-soluble (AS) proteins (chiefly cellular protein) and DNA, when glucose was the only source of nonprotein energy. Intracellular sodium and water content and nonalkali-soluble proteins (largely extracellular proteins) were lower in the skeletal muscle of the amino acid/glucose group than in that of the group receiving the fat regimen. No differences in RNA concentration, RNA/AS protein, or AS protein/DNA ratios were observed. These data suggest that conditions of high insulin production in the postnatal growth period favored increased DNA replication and accretion of AS protein. The differences in water and electrolyte composition indicate that the rate of chemical maturation of skeletal muscle was slower in the piglets receiving amino acids/glucose/fat than in those on the glucose regimen. This study has demonstrated that the source of nonprotein energy can influence skeletal muscle maturation in the postnatal period.