With the replacement of protein hydrolysates with crystalline amino acids, early introduction of amino acids has been advocated recently for preterm infants. However, there are little published data on the use of early amino acids in the very low birth weight (VLBW) infants with birth weights < 1200 grams. Eighteen VLBW preterm infants were selected to receive amino acids within the first 24 hours of life (Early Group) or after 48 hours of life(Control). Both groups received Trophamine as the amino acid solution with the early group starting at 1.5 g/kg/d and the controls starting at 0.5 g/kg/d. The initial total kcal/kg and amount of acetate given were recorded over the next 48 hours. Serum bicarbonate and base excess were recorded during the study period. Plasma amino acid levels were determined in 4 infants in the Early Group. There were no differences in gestational ages (27 ± 1 vs 26 ± 3 weeks) or birth weights (1046 ± 174 vs 913 ± 214 grams) between the groups. The average day of life that amino acids were started on the Early group was day 1 while the controls were started on day 3 of life. The initial total kcal was similar between the groups at 40 kcal/kg/d. During the study, the Early Group exhibited lower serum bicarbonate levels than the controls (16 ± 3 vs 20 ± 2 mmols/L, P<0.02). Also the base excess was lower in the Early Group vs controls (-7.5 ± 2.5 vs -3.1 ±1.6, P<0.001). The Early Group also required more daily acetate than controls, 4.6 ± 1.6 mmols vs 2.1 ± 0.7 mmols, P<0.003. The Early Group had 11 out of 21 amino acids above term breast fed reference values, 4/21 amino acids below reference values, and 6/21 amino acids at the reference values. In summary, the first day administration of amino acids in very low birth weight infants resulted in metabolic acidosis and abnormal amino acid pattern. It is speculated that this metabolic acidosis and amino acid pattern are due to immaturity of metabolic pathways and that the use of amino acids within the first 24 hours of life should be used with caution in VLBW infants.