1. We investigated the effects of starting amino acid administration on post-natal day 2 on protein turnover and nitrogen balance in appropriate-for-gestational-age, very-low-birth-weight infants. Eighteen infants were divided into two groups. Group A received from day 2 onwards an amino acid solution, whereas group B started on this solution after day 4. Both groups were exclusively parenterally fed, 200 kJ day-1 kg-1 on post-natal days 3 and 4. Group A (birth weight 1.5 +/- 0.3 kg) received 4.6 g of glucose, 1.9 g of fat and 2.3 g of amino acids day-1 kg-1 body weight. Group B (birth weight 1.4 +/- 0.2 kg) received 7.0 g of glucose and 1.9 g of fat day-1 kg-1 body weight. 2. At post-natal day 3, a primed constant infusion of 3 mg of [15N]glycine day-1 kg-1 was given. Protein flux, protein synthesis and protein breakdown were calculated from the 15N enrichment in urinary ammonia. In five out of nine infants in group B no plateau of 15N enrichment in urinary urea could be detected, whereas in group A two out of nine infants did not reach a plateau. For this reason we did not use the end product urea for our calculations. 3. The administration of the amino acids resulted in a higher protein flux (6.9 +/- 1.5 g day-1 kg-1 versus 5.2 +/- 0.9 g day-1 kg-1) and a higher protein synthesis rate (6.0 +/- 1.4 g day-1 kg-1 versus 4.6 +/- 0.8 g day-1 kg-1) in group A.(ABSTRACT TRUNCATED AT 250 WORDS)