The plasma amino acid concentrations were measured in 12 neonates with phenylketonuria (PKU). Blood samples were taken when fasting and two hours after a meal. The individual amino acid concentrations, the glycine/valine ratios, and the total amino acid concentrations were mutually compared and also compared to reference intervals. When the diet was instituted and plasma phenylalanine decreased, we observed a normalization of the postprandial response of isoleucine, leucine, tyrosine, and valine.