The amino acid concentrations of rat body protein and rat milk protein were measured at 1, 6, 10 and 16 d of age to determine whether the concentration of each amino acid in proportion to the concentration of total recovered amino acids (amino acid composition) in the body changes during early growth and development and whether parallel changes occur in the amino acid composition of milk. The ratio of essential to nonessential amino acids in bodies decreased 10% during the suckling period (P < 0.05). The ratio of essential to nonessential amino acids in milk proteins decreased by 6% (P < 0.05). With the exception of threonine, the body to milk ratio for each essential amino acid seemed similar to that for the other essential amino acids, implying that most essential amino acids are equally limiting for the synthesis of body protein in exclusively milk-fed pups. Measurements of milk intake and milk composition were used to determine amino acid intakes and hence the efficiency with which milk amino acids were used for body protein deposition. The efficiency of essential amino acid utilization was 75% between 1 and 6 d, 86% between 6 and 10 d, and 62% between 10 and 16 d of age. Although the overall efficiency of nonessential amino acid utilization showed a similar pattern, the results suggested the need for a high degree of glycine and arginine synthesis.