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 [ 15 N]glycine day −1 kg −1 was given. Protein flux, protein synthesis and protein breakdown were calculated from the 15 N enrichment in urinary ammonia. In five out of nine infants in group B no plateau of 15 N 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. There was no statistically significant difference in protein breakdown. The administration of amino acids reversed a negative protein balance (−0.6 ± 0.2 g day −1 kg −1 ) into a positive one (1.4 ± 0.2 g day −1 kg −1 . No adverse effects of the amino acid infusion were seen. 4. We conclude that the early introduction of amino acids has, even at this relatively low energy intake of 200 kJ day −1 kg −1 , a positive effect on protein balance by increasing protein synthesis.