Biochemical maturation accompanies the rapid accretion of skeletal muscle in early life. We wished to determine whether changes in muscle protein accretion, induced by variations in food intake, altered the biochemical maturation of the soleus and the extensor digitorum longus (EDL) muscles. Rat pups were suckled in litters of 4, 10, or 16 to induce differences in food intake. At 21 days of age, muscle protein and DNA were quantitated and biochemical maturation was assessed from measurement of [3H]ouabain-binding site abundance and myosin isoform composition. Differences in food intake produced a twofold range in body and muscle weights and protein and DNA contents. Protein accretion was more sensitive to nutrient intake in the soleus than in the EDL. Serum 3-5,3'-triiodothyronine (T3) and insulin concentrations decreased with a reduction in food intake. Total ouabain-binding sites were not altered in either muscle and were independent of muscle size. Differences in myosin isoform composition were more pronounced for the soleus than the EDL, but were relatively small in magnitude. These results demonstrate that, whereas postnatal muscle protein accretion and circulating hormone concentrations are sensitive to food intake, the biochemical maturation is resilient. The immature muscle does not exhibit the fiber type-specific responses to malnutrition typical of mature muscle.